Level 1
Optics MCQs set 2 for FPSC ASF Junior Airport Security Officer Everyday Science — 20 solved questions.
Q1. Which phenomenon is responsible for the working of optical fibres?
Answer: Total internal reflection
Explanation: In optical fibres, light travels through a dense glass core surrounded by a less dense cladding; when light hits the core-cladding boundary at an angle greater than the critical angle, it undergoes total internal reflection and stays within the core, allowing signals to travel vast distances with minimal loss.
Q2. When light passes from air into glass, which property of light remains unchanged?
Answer: Frequency
Explanation: Frequency is determined by the source of the wave and remains constant as light passes between media; speed and wavelength both change (λ = v/f), but frequency is conserved, which is why colour (frequency) does not change on refraction.
Q3. Which colour of visible light has the highest frequency?
Answer: Violet
Explanation: Violet light has the shortest wavelength (~380-420 nm) in the visible spectrum and therefore the highest frequency among visible colours, since frequency and wavelength are inversely related (c = fλ). Red light is at the opposite end with the longest wavelength and lowest frequency.
Q4. The angle of incidence is always equal to the angle of reflection. This is the law of:
Answer: Reflection
Explanation: The law of reflection states that when a ray of light strikes a reflective surface, the angle of incidence (measured from the normal to the surface) equals the angle of reflection. This law applies to all types of wave reflection and governs the behaviour of mirrors, sonar, and radar systems.
Q5. A concave mirror is also called a:
Answer: Converging mirror
Explanation: A concave mirror curves inward and reflects parallel rays to converge at a focal point in front of the mirror, hence it is called a converging mirror; convex mirrors diverge rays and are called diverging mirrors.
Q6. Snell's Law describes the relationship between the angles of incidence and refraction using the formula:
Answer: n1 sin θ1 = n2 sin θ2
Explanation: Snell's Law states that n₁ sin θ₁ = n₂ sin θ₂, where n represents the refractive index of each medium and θ represents the angle measured from the normal at the interface. This relationship describes how light bends when passing between media of different optical densities.
Q7. When light travels from a denser medium to a rarer medium at a critical angle, the phenomenon that occurs is called:
Answer: Total internal reflection
Explanation: Total internal reflection occurs when light travelling from a denser medium (e.g., glass or water) to a less dense medium (e.g., air) hits the boundary at an angle of incidence equal to or greater than the critical angle, so that no refracted ray exits.
Q8. The focal length of a convex mirror is considered:
Answer: Negative and virtual
Explanation: In the New Cartesian sign convention, a convex mirror has a negative focal length because its principal focus lies behind the reflecting surface on the same side as the object, making it a virtual focus. This virtual focus is why convex mirrors always form diminished, upright, virtual images regardless of object position.
Q9. The refractive index of a medium is defined as the ratio of the speed of light in vacuum to the:
Answer: Speed of light in that medium
Explanation: The refractive index (n) of a medium is defined as the ratio of the speed of light in a vacuum (c) to the speed of light in that medium (v): n = c/v. A higher refractive index means light travels more slowly in the medium and bends more upon entering it.
Q10. The mirror formula relating object distance (u), image distance (v), and focal length (f) is:
Answer: 1/f = 1/v + 1/u
Explanation: The mirror formula is 1/f = 1/v + 1/u, where f is the focal length, v is the image distance, and u is the object distance (all measured from the pole of the mirror). This relationship holds for both concave and convex mirrors when sign conventions are applied consistently.
Q11. The lens formula relating object distance (u), image distance (v), and focal length (f) is:
Answer: 1/f = 1/v - 1/u
Explanation: The standard lens formula is 1/f = 1/v − 1/u, where f is the focal length, v is the image distance, and u is the object distance (negative for a real object using the real-is-positive sign convention). This formula applies to both converging and diverging thin lenses.
Q12. An object placed at the center of curvature of a concave mirror produces an image that is:
Answer: Real, inverted, and the same size as the object
Explanation: When an object is placed at the center of curvature of a concave mirror (distance = 2f), reflected rays converge at the same distance on the other side, producing a real, inverted image that is the same size as the object.
Q13. Light travels from air (n=1) into glass (n=1.5). The light bends:
Answer: Towards the normal
Explanation: When light passes from a less dense medium (air, n=1) to a denser medium (glass, n=1.5), it slows down and bends towards the normal according to Snell's Law: n₁sinθ₁ = n₂sinθ₂. Since n₂ > n₁, the refracted angle is smaller, meaning the ray bends toward the normal.
Q14. The image formed by a convex mirror is always:
Answer: Virtual, erect, and diminished
Explanation: A convex (diverging) mirror always produces a virtual, erect, and diminished image regardless of where the object is placed, because reflected rays diverge and only appear to meet behind the mirror. This property makes convex mirrors useful as rear-view and security mirrors.
Q15. The magnification produced by a spherical mirror is given by the formula m = -v/u. A concave mirror produces an image at v = -30 cm for an object at u = -10 cm. The magnification is:
Answer: -3
Explanation: Using the magnification formula m = −v/u, with v = −30 cm and u = −10 cm, gives m = −(−30)/(−10) = −3. The negative value confirms the image is real and inverted, which is characteristic of a concave mirror when the object is beyond the focal point.
Q16. The phenomenon of light bending when passing from one medium to another is called:
Answer: Refraction
Explanation: Refraction is the bending of light as it crosses the boundary between two media of different optical densities, caused by a change in the wave's speed. Reflection is the bouncing of light off a surface, while diffraction is the spreading of waves around obstacles or through openings.
Q17. Light of wavelength 600 nm travels through glass with refractive index 1.5. The speed of light in this glass is approximately (speed of light in vacuum = 3 × 10⁸ m/s):
Answer: 2.0 × 10⁸ m/s
Explanation: The speed of light in a medium equals c divided by the refractive index: v = (3 × 10⁸) / 1.5 = 2.0 × 10⁸ m/s. A higher refractive index means light travels more slowly through that medium compared to a vacuum.
Q18. A concave lens always produces an image that is:
Answer: Virtual, erect, and diminished
Explanation: A concave (diverging) lens causes light rays to spread outward as if they originate from a virtual focal point on the same side as the object, always forming a virtual, erect, and diminished image regardless of object position.
Q19. The speed of light in a vacuum is:
Answer: 3 × 10⁸ m/s
Explanation: The speed of light in a vacuum is approximately 3 × 10⁸ m/s (more precisely 2.998 × 10⁸ m/s), a fundamental physical constant denoted by c. This value is the universal speed limit and is central to Einstein's theory of special relativity.
Q20. Which type of mirror is used as a rear-view mirror in vehicles?
Answer: Convex mirror
Explanation: Convex mirrors have an outwardly curved reflective surface that diverges incoming light rays, producing a virtual, upright, and diminished image with a wider field of view than a flat mirror of the same size. This wide-angle view makes them ideal as rear-view mirrors, allowing drivers to see more of the road behind them.