Light: Reflection and Refraction
Introduction
Light is a form of energy that enables us to see objects. The chapter Light – Reflection and Refraction deals with the behavior of light when it reflects from mirrors and refracts through transparent media like glass and water.
Reflection of Light
Reflection of light is the phenomenon by which light bounces back after striking a smooth surface such as a mirror.
Laws of Reflection
- The angle of incidence is equal to the angle of reflection.
- The incident ray, reflected ray, and the normal at the point of incidence lie in the same plane.
Spherical Mirrors
A spherical mirror is a mirror whose reflecting surface is part of a sphere.
Types of Spherical Mirrors
- Concave Mirror: Reflecting surface curves inward.
- Convex Mirror: Reflecting surface curves outward.
Important Terms Related to Spherical Mirrors
- Pole (P): Center of the mirror.
- Centre of Curvature (C): Center of the sphere of which mirror is a part.
- Radius of Curvature (R): Distance between P and C.
- Principal Axis: Straight line passing through P and C.
- Principal Focus (F): Point where parallel rays meet or appear to meet.
- Focal Length (f): Distance between P and F.
Mirror Formula
1/f = 1/v + 1/u
Where,
u = object distance
v = image distance
f = focal length
Magnification
m = hi / ho = -v / u
Where,
hi = height of image
ho = height of object
Refraction of Light
Refraction is the bending of light when it passes from one medium to another due to change in speed.
Laws of Refraction (Snell’s Laws)
- The incident ray, refracted ray and normal lie in the same plane.
- The ratio of sine of angle of incidence to sine of angle of refraction is constant.
Refractive Index
n = sin i / sin r
Refractive index indicates how much light bends in a medium.
Refraction Through Glass Slab
- Incident ray and emergent ray are parallel.
- There is lateral displacement of light.
Lenses
A lens is a transparent medium bounded by two surfaces.
Types of Lenses
- Convex Lens: Converging lens
- Concave Lens: Diverging lens
Lens Formula
1/f = 1/v + 1/u
Magnification by Lens
m = v / u
Power of a Lens
P = 1/f (in meters)
Unit: Dioptre (D)
Uses of Mirrors and Lenses
- Concave mirror: headlights, shaving mirrors
- Convex mirror: rear-view mirrors
- Convex lens: magnifying glass, microscope
- Concave lens: spectacles for myopia
Key Points for Exams
- Learn sign convention carefully
- Practice ray diagrams
- Memorize formulas and definitions
- Numericals are very important
1. Light changes its direction when it strikes a smooth surface due to:
a) Refraction
b) Reflection
c) Dispersion
d) Absorption
Answer: b) Reflection
2. The angle between the incident ray and the normal is called:
a) Angle of reflection
b) Angle of refraction
c) Angle of incidence
d) Angle of deviation
Answer: c) Angle of incidence
3. Which mirror is used as a rear-view mirror in vehicles?
a) Plane mirror
b) Concave mirror
c) Convex mirror
d) Parabolic mirror
Answer: c) Convex mirror
4. The image formed by a plane mirror is always:
a) Real and inverted
b) Virtual and erect
c) Real and erect
d) Virtual and inverted
Answer: b) Virtual and erect
5. The focal length of a spherical mirror is equal to:
a) Radius of curvature
b) Twice the radius of curvature
c) Half the radius of curvature
d) One-third of radius of curvature
Answer: c) Half the radius of curvature
6. The mirror that can form both real and virtual images is:
a) Plane mirror
b) Convex mirror
c) Concave mirror
d) Cylindrical mirror
Answer: c) Concave mirror
7. Which mirror always forms a diminished image?
a) Plane mirror
b) Concave mirror
c) Convex mirror
d) Parabolic mirror
Answer: c) Convex mirror
8. The bending of light when it passes from one medium to another is called:
a) Reflection
b) Diffraction
c) Refraction
d) Dispersion
Answer: c) Refraction
9. The ratio of speed of light in vacuum to speed of light in a medium is called:
a) Power
b) Magnification
c) Refractive index
d) Focal length
Answer: c) Refractive index
10. The unit of power of a lens is:
a) Meter
b) Dioptre
c) Centimeter
d) Joule
Answer: b) Dioptre
11. A lens which converges light rays is called:
a) Concave lens
b) Plane lens
c) Convex lens
d) Cylindrical lens
Answer: c) Convex lens
12. A concave lens always forms an image which is:
a) Real and inverted
b) Virtual and erect
c) Real and erect
d) Virtual and inverted
Answer: b) Virtual and erect
13. The magnification produced by a plane mirror is:
a) Less than 1
b) Equal to 1
c) Greater than 1
d) Zero
Answer: b) Equal to 1
14. When light travels from a rarer to a denser medium, it bends:
a) Away from the normal
b) Towards the normal
c) Along the normal
d) Parallel to the surface
Answer: b) Towards the normal
15. The lens used to correct myopia is:
a) Convex lens
b) Concave lens
c) Cylindrical lens
d) Plane lens
Answer: b) Concave lens
16. The mirror formula is:
a) 1/f = v + u
b) 1/f = 1/v − 1/u
c) 1/f = 1/v + 1/u
d) f = v + u
Answer: c) 1/f = 1/v + 1/u
17. When an object is placed at infinity, a convex lens forms the image at:
a) Centre of curvature
b) Principal focus
c) Optical centre
d) Between F and 2F
Answer: b) Principal focus
18. The emergent ray from a rectangular glass slab is:
a) Perpendicular to incident ray
b) Parallel to incident ray
c) Opposite to incident ray
d) At same angle
Answer: b) Parallel to incident ray
19. The power of a lens depends on:
a) Size of lens
b) Thickness of lens
c) Focal length of lens
d) Colour of lens
Answer: c) Focal length of lens
20. The magnification of a concave mirror for a real image is always:
a) Positive
b) Negative
c) Zero
d) One
Answer: b) Negative
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