Optics Answers

Convergence of concave mirror can be decreased by dipping in 

a. Water

b. Oil

c. Both water and oil

d. Cannot be decreased 

A DH GaAs/GaAlAs ILD has a cavity length of 0.5 mm, an effective loss 

coefficient Γ of 1.2mm^-1

, a confinement factor alpha_eff

 of 0.9, and uncoated facet 

reflectivities of 0.35. Calculate the reduction that occurs in the threshold gain 

coefficient when the reflectivity of one of the facets is increased to 1.

Light from a LED enters through the pupil of the eye whose diameter is 6.1 mm. If the intensity is 2.3x10-11 W/m2, calculate the power of light that enters into the eye. If the LED is replaced with a He-Ne LASER ((=632.8 nm) calculate the power if 1.7x1015 photons/s are emitted. Assume that the laser beams diameter and pupil size are the same.

Photons of wavelength 0.8 µm are incident on a p-n photodiode at a rate of 5 x 

10¹⁰ s-1 and on average the electrons are collected at the terminals of the diode at 

the rate of 1.5 x 10¹⁰ s-1. Calculate (a) The quantum efficiency (%) (b) band gap (in 

eV) of the semiconductor material and (c) the mean output photo current for 12 

µW incident optical power. 

Several monochromatic lights pass through double slits with a separation of s 50 3m. The screen to slits distance is 2.5 m. What is/are the width of its central maximum and its corresponding wavelength(s) where the bright spot can occur? [width is expressed in 3 significant figures]

Wavelength (nm) Width (cm)

A. 350 1.75

B. 400 2.00

C. 575 5.75

D. 700 3.50

A diffraction grating produces a third-order bright spot for wavelength 550 nm at 50^o from the central maximum. What is/are the other angle(s) where the bright spot can occur? [angle is expressed in 2 significant figures]

A. 8^o

B. 18^o

C. 27^o

D. 50^o

A monochromatic light passes through a grating to produce a pattern on a screen at a distance from the grating. Choose the correct statement(s) if the number of lines per com of the grating is doubled.

A. The diffraction pattern becomes brighter.

B. The wavelength of the light becomes longer.

C. The distance between bright fringe increases.

D. The number of observable bright fringes increases

Monochromatic light is incident on double slits. Bright and dark fringes are formed on a screen. If the distance between slits is increased,

A. the central maximum becomes wider.

B. the wavelength becomes shorter.

C. the fringe separation will be increased.

D. the width of each fringe becomes narrower

A beam of monochromatic red light is made to pass through a narrow single slit. Choose the CORRECT statement(s) about the produced diffraction pattern on a screen at a distance from the slit.

A. The centre of the pattern is dark.

B. The centre of the pattern is a bright red.

C. A series of rainbow-like fringes will be produced on the screen.

D. A series of alternating bright red and dark fringes will be produced on the screen.