Geometrical Optics studies the behavior of light as it travels in straight lines and interacts with surfaces. This chapter covers reflection and refraction of light, laws governing these phenomena, image formation by mirrors and lenses, and applications in optical instruments such as microscopes and telescopes. It also includes concepts of the refractive index, total internal reflection, and practical uses in devices like periscopes and cameras.
1. Which law states that the angle of incidence is equal to the angle of reflection?
- A. First law of reflection ✅
- B. Snell’s law
- C. Law of refraction
- D. Law of optics
Explanation: The first law of reflection states that the angle of incidence equals the angle of reflection, measured from the normal.
2. In refraction, light bends towards the normal when it:
- A. Enters a denser medium ✅
- B. Enters a rarer medium
- C. Travels in vacuum
- D. Passes through a mirror
Explanation: When light enters a denser medium from a rarer one, its speed decreases and it bends towards the normal.
3. The refractive index is defined as:
- A. Ratio of wavelength in vacuum to that in medium
- B. Ratio of speed of light in vacuum to speed in the medium ✅
- C. Product of speeds in two media
- D. Difference of speeds in two media
Explanation: n = c / v, where c is the speed of light in vacuum and v is the speed in the medium.
4. Which type of lens is thicker at the center than at the edges?
- A. Convex lens ✅
- B. Concave lens
- C. Plano-concave lens
- D. Diverging lens
Explanation: Convex lenses converge light and are thicker in the middle.
5. The image formed by a plane mirror is:
- A. Real, inverted, same size
- B. Virtual, upright, same size ✅
- C. Real, upright, magnified
- D. Virtual, inverted, diminished
Explanation: Plane mirrors always form virtual, upright images of the same size as the object.
6. Which phenomenon enables a prism to split white light into its constituent colors?
- A. Dispersion ✅
- B. Reflection
- C. Diffraction
- D. Polarization
Explanation: Dispersion occurs because different wavelengths of light refract at slightly different angles.
7. In convex lenses, the focal length is measured from:
- A. The edge of the lens
- B. The optical center to the focal point ✅
- C. The curvature center to the object
- D. The axis to the vertex
Explanation: The focal length is the distance from the optical center to the principal focus of the lens.
8. Total internal reflection occurs when light:
- A. Travels from a denser to a rarer medium at an angle greater than the critical angle ✅
- B. Travels from a rarer to a denser medium
- C. Strikes the surface perpendicularly
- D. Passes through a vacuum
Explanation: Total internal reflection only occurs when moving from denser to rarer medium beyond the critical angle.
9. The power of a lens is measured in:
- A. Meters
- B. Diopters ✅
- C. Newtons
- D. Hertz
Explanation: Power P = 100/f (in cm) or P = 1/f (in meters), measured in diopters.
10. A convex mirror always forms an image that is:
- A. Real and inverted
- B. Virtual, upright, and diminished ✅
- C. Virtual and magnified
- D. Real and magnified
Explanation: Convex mirrors diverge light, forming virtual, upright, and smaller images.
11. Which instrument uses total internal reflection to transmit light signals?
- A. Microscope
- B. Telescope
- C. Optical fiber ✅
- D. Camera
Explanation: Optical fibers guide light through total internal reflection for communication.
12. If a ray of light passes through the optical center of a thin lens, it:
- A. Passes undeviated ✅
- B. Gets reflected
- C. Gets dispersed
- D. Gets totally internally reflected
Explanation: A ray through the optical center of a thin lens is undeviated from its path.
13. The mirror used in vehicle side-view mirrors is:
- A. Plane mirror
- B. Convex mirror ✅
- C. Concave mirror
- D. Parabolic mirror
Explanation: Convex mirrors give a wider field of view, making them suitable for side-view mirrors.
14. Snell’s law is expressed as:
- A. n = c/v
- B. n₁ sin i = n₂ sin r ✅
- C. sin i / sin r = v₂ / v₁
- D. n = sin r / sin i
Explanation: Snell’s law relates the angles of incidence and refraction with the refractive indices of two media.
15. A concave lens forms which type of image of a real object?
- A. Real and inverted
- B. Virtual, upright, and diminished ✅
- C. Virtual and magnified
- D. Real and magnified
Explanation: Concave lenses diverge rays, always forming virtual, upright, smaller images.