804 + Interview Questions in OPTICS IN GENERAL AWARENESS Page 9 InterviewSolution

401.	A person moves with a velocity `v` towards a plane mirror. With what velocity does his image move towards him ?
Answer» The image moves towards the person with a velocity `2 v`.

Discussion

402.	An observer moves towards a stationary plane mirror at a speed of 4 ms-1 with what speed – will his image move towards him?(a) 2 ms-1(b) 4 ms-1 (c) 8 ms-1 (d) the image will stay at rest
Answer» (c) 8 ms^-1 Speed of the image towards the observer = 2 x 4 = 8 ms^-1

402.

An observer moves towards a stationary plane mirror at a speed of 4 ms-1 with what speed – will his image move towards him?(a) 2 ms-1(b) 4 ms-1 (c) 8 ms-1 (d) the image will stay at rest

Answer»

(c) 8 ms^-1

Speed of the image towards the observer = 2 x 4 = 8 ms^-1

Discussion

403.	What is angle of minimum deviation?
Answer» The minimum value of the angle of deviation suffered by a ray on passing through a prism is called the angle of minimum deviation.

Discussion

404.	As per recent development, what is the nature of light? Wave optics and particle nature of light are used to explain which phenomena of light respectively?
Answer» 1. As per recent development, it is now an established fact that light possesses dual nature. Light consists of energy carrier photons. These photons follow the rules of electromagnetic waves. 2. Wave optics explains the phenomena of light such as, interference, diffraction, polarisation, Doppler effect etc. 3. Particle nature of light can be used to explain phenomena like photoelectric effect, emission of spectral lines, Compton effect etc.

404.

As per recent development, what is the nature of light? Wave optics and particle nature of light are used to explain which phenomena of light respectively?

Answer»

1. As per recent development, it is now an established fact that light possesses dual nature. Light consists of energy carrier photons. These photons follow the rules of electromagnetic waves.

2. Wave optics explains the phenomena of light such as, interference, diffraction, polarisation, Doppler effect etc.

3. Particle nature of light can be used to explain phenomena like photoelectric effect, emission of spectral lines, Compton effect etc.

Discussion

405.	When diameter of objective of an astronomical telescope is doubled, its limit of resolution isA. (a) doubledB. (b) quardrupledC. ( c) halvedD. unaffected
Answer» Correct Answer - C

Discussion

406.	If the velocity and wavelength of light in air is Va and λa and that in water is Va and λw , then the refractive index of water is,(a) \(\frac{V_w}{V_a}\)(b) \(\frac{V_a}{V_w}\)(c) \(\frac{λ_w}{λ_a}\)(d) \(\frac{V_a λ}{V_w λ_w}\)
Answer» Correct answer is (b) \(\frac{V_a}{V_w}\)

Discussion

407.	Stars twinkle due to (a) reflection (b) total internal reflection (c) refraction (d) polarisation
Answer» Correct answer is (c) refraction

Discussion

408.	How does a rainbow form?
Answer» 1. The rainbow appears in the sky after a rainfall. 2. Water droplets present in the atmosphere act as small prism. 3. When sunlight enters these water droplets it gets refracted and dispersed. 4. This dispersed light gets totally reflected inside the droplet and again is refracted while coming out of the droplet. 5. As a combined effect of all these phenomena, the seven coloured rainbow is observed.

408.

How does a rainbow form?

Answer»

1. The rainbow appears in the sky after a rainfall.

2. Water droplets present in the atmosphere act as small prism.

3. When sunlight enters these water droplets it gets refracted and dispersed.

4. This dispersed light gets totally reflected inside the droplet and again is refracted while coming out of the droplet.

5. As a combined effect of all these phenomena, the seven coloured rainbow is observed.

Discussion

409.	What is total internal reflection?
Answer» For angles of incidence larger than the critical angle, the angle of refraction is larger than 90°. Thus, all the incident light gets reflected back into the denser medium. This is called total internal reflection.

Discussion

410.	Why are danger signals red in colour ?
Answer» As `lambda_( r)` is largest in the visible region, therfore, red colour is least scattered. Hence it can be seen from large distance. That is why danger signals are red.

Discussion

411.	The deviation through a prism is minimum when……….. .
Answer» Correct Answer - `i_(1) = i_(2)`

Discussion

412.	The diameter of a plano convex lens is `6 cm` and thickness at the centre is `3 mm`. If the speed of light in the material of the lens is `2 xx 10^8 m//s`, what is the focal length of the lens ?A. `15 cm`B. `20 cm`C. `30 cm`D. `10cm`
Answer» Correct Answer - C Here, `d = 6 cm, h = 3 mm = 0.3 cm`, `v = 2 xx 10^(8)m//s, f = ?` If `R` is radius of curvature of the convex surface of lens, then from `((d)/(2))^(2) = (2 R - h)h = 2 R h` `(because h lt lt R)` `R = (d^(2)//4)/(2h) = (6^(2)//4)/(2 xx 0.3) = 15 cm` `mu = ( c)/(v) = (3 xx 10^(8))/(2 xx 10^(8)) = (3)/(2)` From `(1)/(f) = (mu - 1)((1)/(R_(1)) - (1)/(R_(2)))` `= ((3)/(2) - 1)((1)/(oo) + (1)/(15)) = (1)/(30)` `f = 30 cm`

Discussion

413.	Two coherent monochromatic light beams of intensities I and 4I are supperposed. The maximum and minimum possible intensities in the resulting beams are(a) 5I and I (b) 9I and I (c) 5I and 3I (d) 9I and 3I
Answer» (b) 9I and I I_max = \(({\sqrt I_1}+{\sqrt I_2})^2\) = ( \(({\sqrt 41}+{\sqrt I})^2\)) = 9I I_max = \(({\sqrt I_1}+{\sqrt I_2})^2\) = ( \(({\sqrt 41}+{\sqrt I})^2\)) = I

413.

Two coherent monochromatic light beams of intensities I and 4I are supperposed. The maximum and minimum possible intensities in the resulting beams are(a) 5I and I (b) 9I and I (c) 5I and 3I (d) 9I and 3I

Answer»

(b) 9I and I

I_max = \(({\sqrt I_1}+{\sqrt I_2})^2\) = ( \(({\sqrt 41}+{\sqrt I})^2\)) = 9I

I_max = \(({\sqrt I_1}+{\sqrt I_2})^2\) = ( \(({\sqrt 41}+{\sqrt I})^2\)) = I

Discussion

414.	(a) A person can see clearly upto `80 cm`. He uses spectacles of `-0.80 dioptre`, how far can he see clearly ? (b) If a person uses spectacles of power `+ 1.0 dioptre`, what is the nearest distance of distinct vision for him ? Given that near point of the person is `75 cm` from the eye.
Answer» (a) Here, `P = - 0.80 dioptre` `f = (100)/(P) = (100)/(-0.8) = - 125 cm` `v = - 80 cm, u = ?` As `(1)/(v) - (1)/(u) = (1)/(f)` `:. (1)/(u) = (1)/(v) - (1)/(f) = (1)/(-80) + (1)/(125)` `= (-25 + 16)/(2000) = (-9)/(2000)` `u = - (2000)/(9) = - 222.22 cm` The person can see objects upto `222.22 cm`. (b) Here, `P = 1.0 dioptre` `f = (100)/(P) = (100)/(1) = 100 cm` `u = ? v = - 80 cm`, As `(1)/(v) - (1)/(u) = (1)/(f)` `:. (1)/(u) = (1)/(v) - (1)/(f) = -(1)/(75) - (1)/(100) = (-4 - 3)/(300)` `u = -(300)/(7) = - 42.9 cm`. `:.` The nearest distance of distinct vision of the person is `42.9 cm`

Discussion

415.	What si a wavelength ?
Answer» A wavefront is the locus of all such particles of the medium, which are vibrating in the same phase.

Discussion

416.	Light of wavelength `5890 Å` travelling in air enters water of `mu = 4//3`. What will be the frequency and wavelength of light in water ?
Answer» Here, `lambda_(1) = 5890 Å = 5890 xx 10^(-10)m` `mu = 4//3, v_(2) = ? Lambda_(2) = ?` As frequency in water = frequency in air, `:. v_(2) = v_(1) = ( c)/(lambda_(1)) = (3 xx 10^(8))/(5890 xx 10^(-10)) = 5.093 xx 10^(14)c//s` As `mu = (lambda_(1))/(lambda_(2))` `:. lambda_(2) = (lambda_(1))/(mu) = (5890 xx 10^(-10))/(4//3) = 4417.5 xx 10^(-10)m = 4417.5 Å`

Discussion

417.	A light wave enters into glass from water. How are its energy and frequency affected ?
Answer» Energy decreases because some of the energy is reflected back into water. The frequency remains the same.

Discussion

418.	Light waves from two coherent sources having intensities `I and 2 I` cross each other at a point with a phase diff. of `60^(@)`. What is the resultant intensity at the point ? If the sources were incoherent, what would be the resultant intensity ?
Answer» Here, `I_(1) = I and I_(2) = 2 I, phi = 60^(@)` When sources are coherent Amplitude `R` of resultant wave is `R = sqrt(a^(2) + b^(2) + 2 ab cos phi)` `R^(2) = a^(2) + b^(2) + 2 ab cos phi` As intensity `prop ("amplitude")^(2)` `:.` Resultant intensity, `I_(R ) = I_(1) + I_(2) + 2sqrt(I_(1)I_(2)) cos phi = I + 2 I + 2 sqrt(I xx 2I) xx cos 60^(@)` `= 3 I + I sqrt(2) = I(3 + sqrt(2)) = 4.414 I` If the sources are incoherent, `I_(R ) = I_(1) + I_(2) = I + 2 I = 3 I`

Discussion

419.	What is the relation between path diff. and wavelength for constructive interference of two waves ?
Answer» Path diff. `= n lambda`, where `n = 0, 1, 2, …..` i.e., path difference should either be zero or an integral multiple of wavelength.

Discussion

420.	Over a given wavefront, is the amplitude constant ?
Answer» Yes, provided the medium is homogeneous.

Discussion

421.	`A` and `B` are two points on water surface where waves are generated. What is the phase diff. if (i) `A` and `B` are on same wavefront separated by distance `lambda`. (ii) A and `B` are on successive crests separated by distance `2 lambda`. (iii) A and B are on successive troughs separated by distance `3 lambda`.
Answer» (i) Zero (ii) `2pi` radian (iii) `2pi` radian.

Discussion

422.	Why cannot we obtain interference using two independent source of light ?
Answer» The light from an ordinary electric lamp undergoes about 100 million `(10^(8))` random changes every second. Therefore, light from two independent light bulbs, the given set of interference conditions last for only about `10^(-8) second`. As human eye cannot follow such a rapid change, therefore, no interference effects are observed from two independent light bulbs.

Discussion

423.	In a moving car, radio signals are interrupted sometimes. Why ?
Answer» While we are in a moving car, there are periodic interuptions in a radio signal that we hear. It occurs an account of interfering radio waves. This is a common form of interference, called multipath interference between a radio wave and its reflection.

Discussion

424.	Light of wavelength `500 nm` is incident on two slits separated by `1 mm` in YDSE. What is the width of each dark band if the screen is `1 metre` away from the slits ?
Answer» Here, `lambda = 500 nm = 500 xx 10^(-9)m , d = 1 mm = 10^(-3)m , beta = ? D = 1m` `beta = (lambda D)/(d) = (500 xx 10^(-9) xx 1)/(10^(-3)) = 5 xx 10^(-4)m = 0.5 mm`

Discussion

425.	Which of the two colours red and violet travels slower in glass prism.
Answer» Violet colour travels slower. This is because `mu_(v) gt mu_( r) and v = c//mu`.

Discussion

426.	How is a wavefront related to the direction of corresponding rays ?
Answer» Wavefront is always normal to the rays correcponding to it.

Discussion

427.	In YDSE, slits are separated by `0.24 mm` and the screen is kept `160 cm` away from slits. If fringe width is measured to be `0.4 cm`, calculate the wavelength of light used.
Answer» Here, `d = 0.24 mm = 0.24 xx 10^(-3) m` `D = 160 cm = 1.60 m`, `beta = 0.4 cm = 0.4 xx 10^(-2)m, lambda = ?` From `beta = (lambda D)/(d)` `lambda = (beta xx d)/(D) = (0.4 xx 10^(-2) xx 0.24 xx 10^(-3))/(1.6)` `= 6 xx 10^(-7)m`

Discussion

428.	In YDSE, having slits of equal width, let `beta` be the fringe width and `I_(0)` be the maximum intensity. At a distance x from the central brigth fringe, the intensity will beA. `I_(0)cos(x/(beta))`B. `I_(0)cos^(2)(x/(beta))`C. `I_(0)cos^(2)((pix)/(beta))`D. `((I_(0))/4)cos^(2)((pix)/(beta))`
Answer» Correct Answer - C

Discussion

429.	What is the phase difference between any two points on a wavefront ?
Answer» Phase difference between any two points on a wavefront is zero.

Discussion

430.	In YDSE, when separation between two slits is increased, fringe widthA. decreasesB. increasesC. remains the sameD. none of these
Answer» Correct Answer - A

Discussion

431.	The phase difference between two points `A` and `B` on a wavefront separated by distance `lambda` isA. `0`B. `2 pi`C. `pi`D. `pi//2`
Answer» Correct Answer - A

Discussion

432.	Which of the following lens would you prefer to use while reading small letters found in a dictionary? (a) A convex lens of focal length 5 cm (b) A concave lens of focal length 5 cm (c) A convex lens of focal length 10 cm (d) A concave lens of focal length 10 cm
Answer» (d) A concave lens of focal length 10 cm

Discussion

433.	Why do we prefer a maginfying glass of smaller focal length ?
Answer» This is because the nagnifying power is more when focal length is short.

Discussion

434.	What do you understand by normal adjustment of a telescope ?
Answer» In normal adjustment, the final image is formed at infinity.

Discussion

435.	What is the distance between objective and eye lens of telescope in normal adjustment ?
Answer» `x = (f_(0) + f_(e))`.

Discussion

436.	Objective of a compound microscope should have small focal length. Why ?
Answer» To have larger magnifying power.

Discussion

437.	why should objective of a telescope have larger focal length ?
Answer» To obtain larger magnifying power and better intensity of image.

Discussion

438.	Statement-1 : Raman spectrum of a liquid contains lines, whose wavelengths are equal to the incident radiation. Statement-2 : If a photon strikes an atom or a molecule in a liquid, the photon may lose some energy.A. Statement-1 is true, Statement-2 is true. Statement-2 is correct explanation of Statement-1.B. Statement-1 is true, Statement-2 is true, but Statement-2 is not a correct explanation of Statement-1.C. Statement-1 is true, Statement-2 is false.D. Statement-1 is fasle, Statement-2 is true.
Answer» Correct Answer - B Both the statement are true, but statement-2 is not a correct explanation of statement-1.

Discussion

439.	If the energy of the incident and the scattered beam of light are not the same, then it is called as _____. (a) Elastic (b) Raman (c) Inelastic (d) Mie.
Answer» (c) Inelastic

Discussion

440.	State the factor on which refractive index of a medium depend ?
Answer» Refractive index of a medium depends on (i) nature of medium (ii) wavelength of light used (iii) temperature (iv) nature of surrounding medium.

Discussion

441.	On what factors lateral shift of a ray on passing through a glass slab depend ?
Answer» The lateral shift depend on (i) angle of incidence (ii) refractive index of the medium (iii) thickness of refracting medium.

Discussion

442.	For which medium is refractive index maximum ?
Answer» `mu` is maximum `(= 2.46)` for diamond.

Discussion

443.	Consider interference between waves from two sourceof intensities `I and 4 I`. Find intensities at points where phase difference is (i) `pi//2` (ii) `pi`.
Answer» We know `R^(2) = a^(2) + b^(2) + 2 ab cos phi` `:. I_(R ) = I_(1) + I_(2) + 2 sqrt(I_(1)) sqrt(I_(2)) cos phi`, In case (i), `phi = pi//2` `:. I_(R ) = I + 4 I + 2sqrt(I xx 4 I) cos pi//2, I_(R ) = 5 I` In case (ii), `phi = pi` ltbr. `:. I_(R ) = I + 4 I + 2sqrt(I xx 4 I) cos pi = I`

Discussion

444.	What are coherent sources of light ? Why nointerference pattern is observed when two coherent sources are (i) too close (ii) very far apart ?
Answer» As fringe width `beta prop (1)/(d)`, therefore, when d is so large, the width may reduce beyond the visible region. Hence the pattern will not be seen.

Discussion

445.	A glass slab is placed in the path of a beam of convergent light. The point of convergence of light(A) moves towards the glass slab. (B) moves away from the glass slab. (C) remains at the same point. (D) undergoes a lateral shift.
Answer» (A) moves towards the glass slab.

Discussion

446.	In case of a convex mirror, the image formed is(A) always on opposite side, virtual, erect. (B) always on the same side, virtual, erect. (C) always on opposite side, real, inverted. (D) dependent on object distance.
Answer» (A) always on opposite side, virtual, erect.

Discussion

447.	The degree of polarization of partially polarized light is `P = 0.25`. Find the ratio of intensities of the polarized component of this light and the natural component.
Answer» Suppose the partially polarized consists of natural light of intensity `I_(1)` and plane polarized light of intensity `I_(2)` with direction of vibration parallel to, say, `x-axis`. Then when a polaroid is used to transmit, it the light transmitted will have a maximum intensity `(1)/(2)I_(1) + I_(2)`, when the principle direction of the polaroid is parallel to `x-`axis, and will ahve a minimum intensity `(1)/(2)I_(1)` when the principle direction is `_\|_^(r )` to `x`-axis. Thus `P = (I_(max) - I_(min))/(I_(max) + I_(min)) = (I_(2))/(I_(1) + I_(2))` so `(I_(2))/(I_(1)) = (P)/(1- P) = (0.25)/(0.75) = (1)/(3)`.

Discussion

448.	A point object is held between two plane mirrors inclined at `45^(@)`. What is the number of images seen ?
Answer» Correct Answer - `7` Here, `theta = 45^(@)` `n = (360^(@))/(theta) - 1 = (360)/(45) - 1 = 8 - 1 = 7`

Discussion

449.	If mirrors are inclined to each other at an angle of 90°, the total number of images seen for a symmetric position of an object will be (A) 3 (B) 4 (C) 5 (D) 3 or 4
Answer» Correct option is: (A) 3

Discussion

450.	Does the speed of light in vaccume depend upon relative motion between source and observer ?
Answer» No, speed of light in vaccume is an absolute constant.

Discussion

Explore topic-wise InterviewSolutions in .

A person moves with a velocity `v` towards a plane mirror. With what velocity does his image move towards him ?

An observer moves towards a stationary plane mirror at a speed of 4 ms-1 with what speed – will his image move towards him?(a) 2 ms-1(b) 4 ms-1 (c) 8 ms-1 (d) the image will stay at rest

What is angle of minimum deviation?

As per recent development, what is the nature of light? Wave optics and particle nature of light are used to explain which phenomena of light respectively?

When diameter of objective of an astronomical telescope is doubled, its limit of resolution isA. (a) doubledB. (b) quardrupledC. ( c) halvedD. unaffected

If the velocity and wavelength of light in air is Va and λa and that in water is Va and λw , then the refractive index of water is,(a) \(\frac{V_w}{V_a}\)(b) \(\frac{V_a}{V_w}\)(c) \(\frac{λ_w}{λ_a}\)(d) \(\frac{V_a λ}{V_w λ_w}\)

Stars twinkle due to (a) reflection (b) total internal reflection (c) refraction (d) polarisation

How does a rainbow form?

What is total internal reflection?

Why are danger signals red in colour ?

The deviation through a prism is minimum when……….. .

The diameter of a plano convex lens is `6 cm` and thickness at the centre is `3 mm`. If the speed of light in the material of the lens is `2 xx 10^8 m//s`, what is the focal length of the lens ?A. `15 cm`B. `20 cm`C. `30 cm`D. `10cm`

Two coherent monochromatic light beams of intensities I and 4I are supperposed. The maximum and minimum possible intensities in the resulting beams are(a) 5I and I (b) 9I and I (c) 5I and 3I (d) 9I and 3I

(a) A person can see clearly upto `80 cm`. He uses spectacles of `-0.80 dioptre`, how far can he see clearly ? (b) If a person uses spectacles of power `+ 1.0 dioptre`, what is the nearest distance of distinct vision for him ? Given that near point of the person is `75 cm` from the eye.

What si a wavelength ?

Light of wavelength `5890 Å` travelling in air enters water of `mu = 4//3`. What will be the frequency and wavelength of light in water ?

A light wave enters into glass from water. How are its energy and frequency affected ?

Light waves from two coherent sources having intensities `I and 2 I` cross each other at a point with a phase diff. of `60^(@)`. What is the resultant intensity at the point ? If the sources were incoherent, what would be the resultant intensity ?

What is the relation between path diff. and wavelength for constructive interference of two waves ?

Over a given wavefront, is the amplitude constant ?

Why cannot we obtain interference using two independent source of light ?

In a moving car, radio signals are interrupted sometimes. Why ?

Light of wavelength `500 nm` is incident on two slits separated by `1 mm` in YDSE. What is the width of each dark band if the screen is `1 metre` away from the slits ?

Which of the two colours red and violet travels slower in glass prism.

How is a wavefront related to the direction of corresponding rays ?

In YDSE, slits are separated by `0.24 mm` and the screen is kept `160 cm` away from slits. If fringe width is measured to be `0.4 cm`, calculate the wavelength of light used.

In YDSE, having slits of equal width, let `beta` be the fringe width and `I_(0)` be the maximum intensity. At a distance x from the central brigth fringe, the intensity will beA. `I_(0)cos(x/(beta))`B. `I_(0)cos^(2)(x/(beta))`C. `I_(0)cos^(2)((pix)/(beta))`D. `((I_(0))/4)cos^(2)((pix)/(beta))`

What is the phase difference between any two points on a wavefront ?

In YDSE, when separation between two slits is increased, fringe widthA. decreasesB. increasesC. remains the sameD. none of these

The phase difference between two points `A` and `B` on a wavefront separated by distance `lambda` isA. `0`B. `2 pi`C. `pi`D. `pi//2`

Which of the following lens would you prefer to use while reading small letters found in a dictionary? (a) A convex lens of focal length 5 cm (b) A concave lens of focal length 5 cm (c) A convex lens of focal length 10 cm (d) A concave lens of focal length 10 cm

Why do we prefer a maginfying glass of smaller focal length ?

What do you understand by normal adjustment of a telescope ?

What is the distance between objective and eye lens of telescope in normal adjustment ?

Objective of a compound microscope should have small focal length. Why ?

why should objective of a telescope have larger focal length ?

If the energy of the incident and the scattered beam of light are not the same, then it is called as _____. (a) Elastic (b) Raman (c) Inelastic (d) Mie.

State the factor on which refractive index of a medium depend ?

On what factors lateral shift of a ray on passing through a glass slab depend ?

For which medium is refractive index maximum ?

Consider interference between waves from two sourceof intensities `I and 4 I`. Find intensities at points where phase difference is (i) `pi//2` (ii) `pi`.

What are coherent sources of light ? Why nointerference pattern is observed when two coherent sources are (i) too close (ii) very far apart ?

A glass slab is placed in the path of a beam of convergent light. The point of convergence of light(A) moves towards the glass slab. (B) moves away from the glass slab. (C) remains at the same point. (D) undergoes a lateral shift.

In case of a convex mirror, the image formed is(A) always on opposite side, virtual, erect. (B) always on the same side, virtual, erect. (C) always on opposite side, real, inverted. (D) dependent on object distance.

The degree of polarization of partially polarized light is `P = 0.25`. Find the ratio of intensities of the polarized component of this light and the natural component.

A point object is held between two plane mirrors inclined at `45^(@)`. What is the number of images seen ?

If mirrors are inclined to each other at an angle of 90°, the total number of images seen for a symmetric position of an object will be (A) 3 (B) 4 (C) 5 (D) 3 or 4

Does the speed of light in vaccume depend upon relative motion between source and observer ?