Free Optics MCQs with Answers

• A. 3 × 10⁶ m/s
• B. 3 × 10⁸ m/s
• C. 3 × 10¹⁰ m/s
• D. 3 × 10⁵ m/s

Explanation: Light travels at approximately 3 × 10⁸ metres per second in a vacuum — about 300,000 kilometres per second.

• A. towards the normal
• B. away from the normal
• C. along the normal
• D. not at all

Explanation: The speed of light increases in the less dense medium, so the ray bends away from the normal, and beyond the critical angle it cannot…

• A. the speed of light in the medium divided by the speed in a vacuum
• B. the speed of light in a vacuum divided by the speed in the medium
• C. the angle of incidence divided by the angle of refraction
• D. the wavelength in the medium divided by that in a vacuum

Explanation: Since light always travels slower in a material than in a vacuum, the refractive index is always greater than one, being about 1.33 for…

• A. a less dense to a denser medium at any angle
• B. a denser to a less dense medium at an angle greater than the critical angle
• C. a vacuum into glass
• D. any medium at normal incidence

Explanation: Both conditions are needed: the second medium must be optically less dense, and the angle of incidence must exceed the critical angle…

• A. repeated total internal reflection at the core cladding boundary
• B. repeated refraction out of the fibre
• C. conducting electricity
• D. absorbing and re-emitting the light

Explanation: The core has a higher refractive index than the cladding, so light striking the boundary beyond the critical angle is reflected without…

• A. beyond 2F
• B. between F and 2F
• C. between the lens and F
• D. at F

Explanation: With the object between one and two focal lengths from the lens, the image forms beyond 2F on the other side, real, inverted and larger…

• A. the focal length in metres
• B. the reciprocal of the focal length in metres
• C. the focal length in centimetres
• D. the magnification produced

Explanation: A lens of focal length 0.5 m has a power of 2 dioptres, and a shorter focal length means a more strongly converging lens.

• A. behind the retina, and is corrected with a converging lens
• B. in front of the retina, and is corrected with a diverging lens
• C. on the retina but upside down
• D. outside the eye

Explanation: The eyeball is too long or the lens too strong, so parallel rays converge before reaching the retina, and a diverging lens spreads them…

• A. the slit separation is increased
• B. the wavelength is decreased
• C. the screen is moved further away
• D. the slits are made narrower only

Explanation: Fringe spacing is the wavelength multiplied by the screen distance divided by the slit separation, so a more distant screen or a longer…

• A. the prism adds colour to the light
• B. different wavelengths travel at different speeds in glass and so are refracted through different angles
• C. the glass absorbs some colours
• D. light slows down equally for all colours

Explanation: Dispersion arises because the refractive index of glass depends slightly on wavelength, being greatest for violet, which is therefore…

• A. real, inverted and the same size
• B. virtual, upright, laterally inverted and the same size as the object
• C. real, upright and magnified
• D. virtual and diminished

Explanation: The rays only appear to come from behind the mirror, so the image cannot be caught on a screen and is virtual, and it lies as far behind…

• A. real and inverted
• B. virtual, upright and magnified
• C. virtual and diminished
• D. real and diminished

Explanation: Inside the focal length the reflected rays diverge, so they appear to come from an enlarged upright image behind the mirror.

• A. 30 degrees
• B. 42 degrees
• C. 60 degrees
• D. 90 degrees

Explanation: The sine of the critical angle equals one over the refractive index, so sin C is 0.667 and C is about 42 degrees, the standard value for…

• A. increasing the magnification only
• B. increasing the diameter of its objective lens or mirror
• C. using a longer eyepiece
• D. using light of longer wavelength

Explanation: Resolution is limited by diffraction at the aperture, so a larger objective produces a smaller diffraction disc and separates closer…

• A. 750 nm
• B. 333 nm
• C. 500 nm
• D. 250 nm

Explanation: Frequency is unchanged on entering a medium, and since the speed falls by the factor of the refractive index the wavelength must fall in…

• A. much larger than the wavelength
• B. comparable to the wavelength of the light
• C. much smaller than an atom
• D. irrelevant to the wavelength

Explanation: Light bends appreciably round an obstacle only when the obstacle is of the same order of size as its wavelength, and since visible…

• A. virtual and diminished
• B. real, inverted and magnified, which the eyepiece then magnifies further
• C. real and diminished
• D. at infinity

Explanation: The specimen sits just beyond the objective's focal point, so the objective produces a real magnified image inside the tube, and the…

• A. the atmosphere is blue in colour
• B. shorter wavelengths are scattered more strongly by air molecules
• C. the sea reflects onto the sky
• D. the Sun emits mostly blue light

Explanation: Rayleigh scattering varies as the inverse fourth power of wavelength, so blue is scattered far more than red and reaches the eye from all…

• A. partially polarised in the horizontal plane, which the filter blocks
• B. of a different frequency
• C. travelling faster
• D. already absorbed by the surface

Explanation: Reflection off water or a road partially polarises the light horizontally, so a filter oriented to transmit only vertical vibrations…

• A. 5 cm
• B. 25 cm
• C. 100 cm
• D. infinity

Explanation: About 25 cm is the closest an object can be brought while remaining in sharp focus without strain, and it is the reference distance used…