Two Polaroids are placed one behind the other. The light coming out is completely cut off when the angle between their planes of transmission is
- A.0 degrees
- B.45 degrees
- C.60 degrees
- D.90 degrees
Correct answer
Explanation
The correct answer is D, 90 degrees. By Malus's law the intensity transmitted by the second Polaroid is the intensity falling on it multiplied by the square of the cosine of the angle between the two transmission planes. At 90 degrees the cosine is zero, so no light emerges and the Polaroids are said to be crossed. Option A is wrong because at zero degrees the planes are parallel and the transmitted intensity is maximum. Option B is wrong because at 45 degrees the square of the cosine is one half, so half the light gets through. Option C is wrong because at 60 degrees the square of the cosine is one quarter, so a quarter of the light still passes. This behaviour is the standard laboratory test for plane polarised light and is also how a liquid crystal display controls brightness.
Read the full article: Interference, Diffraction and Polarisation of Light Notes
Practice Questions
View allBrewster's law is (where the symbol mu stands for the refractive index and i_p for the polarising angle)
- A.i_p = 90 degrees
- B.mu = tan i_p
- C.i_p minus r = tan mu
- D.i_p = tan mu
Show answer
Brewster found that when unpolarised light falls on a transparent surface at one particular angle of incidence, called the polarising angle, the reflected light is completely plane polarised, and at that angle the refractive index of the surface equals the tangent of the polarising angle.
Read the full explanation →The headlights of a distant car are 1.4 m apart. If the diameter of the pupil of the eye is 3 mm, what is the maximum distance at which the two headlights can be resolved? Consider the headlights as point sources of wavelength 500 nm.
- A.3448 m
- B.68.96 m
- C.34.48 m
- D.6896 m
Show answer
By the Rayleigh criterion the smallest angle the eye can resolve is 1.22 times the wavelength divided by the diameter of the pupil, that is 1.22 multiplied by 500 nanometres and divided by 3 millimetres, which comes to about 2.03 times ten to the power minus four radian.
Read the full explanation →When a thin glass plate of refractive index 1.5 is introduced in one of the arms of a Michelson interferometer using light of wavelength 5890 angstrom, there is a shift of 10 fringes. The thickness of the plate is
- A.5.89 m
- B.5.89 times ten to the power minus three metre
- C.5.89 times ten to the power minus six metre
- D.5.89 times ten to the power minus four metre
Show answer
Inserting a plate of thickness t in one arm makes the light cross it twice, so the extra path introduced is twice the thickness times the quantity refractive index minus one.
Read the full explanation →The phenomenon that establishes the transverse nature of light waves is
- A.Interference
- B.Diffraction
- C.Polarisation
- D.Dispersion
Show answer
Only a transverse wave, in which the vibrations are at right angles to the direction of travel, can have those vibrations restricted to a single plane by a Polaroid.
Read the full explanation →The brilliant colours seen in a soap bubble or in a thin film of oil spread on water are caused by
- A.Interference of light reflected from the two surfaces of the film
- B.Total internal reflection inside the film
- C.Dispersion of light as in a prism
- D.Scattering of light by the molecules of the film
Show answer
The light reflected from the upper surface of the thin film and the light reflected from the lower surface travel slightly different distances and interfere.
Read the full explanation →