Free Optics MCQs with Answers

1. What is the approximate speed of light in a vacuum?

• 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. This is the maximum speed at which any information can travel, and light slows down in denser media such as glass or water.

2. Light travelling from a denser medium into a less dense one bends

• 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 escape at all and is totally internally reflected. Going the other way, from less dense to denser, the ray bends towards the normal. A ray along the normal passes straight through whatever the media.

3. The refractive index of a medium is defined as

• 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 water and 1.5 for glass. Snell's law relates the sines of the angles, not the angles themselves, which is the trap in the third option. The frequency stays the same on refraction while the wavelength shortens.

4. Total internal reflection can occur only when light travels from

• 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, which is about 42 degrees for glass to air. This is the principle behind optical fibres, prismatic binoculars and the sparkle of a cut diamond. At normal incidence there is no refraction to suppress.

5. An optical fibre transmits light along its length by

• 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 loss and zigzags along even when the fibre bends. This carries far more information than a copper cable and is immune to electrical interference. Endoscopes use bundles of such fibres to see inside the body.

6. A converging lens forms a real, inverted and magnified image when the object is placed

• 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, which is the arrangement used in a projector. Beyond 2F the image would be diminished, as in a camera. Inside F the image becomes virtual, upright and magnified, which is the magnifying glass.

7. The power of a lens is measured in dioptres and is equal to

• 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. Diverging lenses have negative focal lengths and therefore negative powers, which is how spectacle prescriptions for short sight are written. Powers of lenses in contact simply add, which is why the unit is convenient.

8. A short sighted person cannot see distant objects clearly because the image forms

• 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 slightly first to push the focus back. Long sight is the opposite case, corrected with a converging lens. Both defects are described by the far point and near point of the eye.

9. In Young's double slit experiment, the fringe spacing increases if

• 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 wavelength widens the fringes while a larger slit separation narrows them. This is why red fringes are wider than blue ones in the same apparatus. The experiment is the classic demonstration of the wave nature of light.

10. White light passing through a prism is split into colours because

• 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 deviated most and red least. The prism separates colours already present in white light rather than creating them, as Newton demonstrated by recombining them with a second prism. The same effect produces a rainbow in water droplets.