Light: Reflection and Refraction
1. Ray of Light
A line drawn in the direction of propagation of light is called a ray of light.
2. Beam of Light
A group of rays of light emitted by a source of light is called a beam of light. A light beam is of three types:
- Parallel beam: A group of light rays parallel to each other.
- Divergent beam: A group of light rays spreading out from a source of light.
- Convergent beam: A group of light rays meeting at a point.
3. Reflection of Light
Some surfaces have the ability to send the light back in the same medium when light strikes them. This phenomenon is called reflection of light.
- The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.
- The angle of incidence is always equal to the angle of reflection, ∠i = ∠r.
4. Image
When light rays meet or appear to meet after reflection from a mirror, an image is formed.
- Real Image: Formed by actual intersection of light rays after reflection.
- Virtual Image: Formed by producing the reflected rays backward after reflection.
5. Plane Mirror
A piece of glass whose one side is polished with silver paint and covered by a protective red paint layer.
6. Spherical Mirrors
Parts of hollow glass spheres with one surface polished. There are two types:
- Concave Mirror: Outer surface polished; inner surface reflects light.
- Convex Mirror: Inner surface polished; outer surface reflects light.
7. Principal Focus
The point on the principal axis of a spherical mirror where parallel light rays meet or appear to meet after reflection.
8. Focal Length
The distance between the pole (P) and principal focus (F) of a spherical mirror, denoted by f.
9. Uses of Concave Mirror
- Used as a shaving mirror for magnified reflections.
- Used in solar heating devices like solar cookers.
- Used for security checking.
10. Uses of Convex Mirror
- Used as rear-view mirrors in automobiles for a wider field of view.
- Used in street lights.
11. Mirror Formula
A relation between object distance (u), image distance (v), and focal length (f).
12. Magnification
Defined as the ratio of the height of the image to the height of the object (m = h2 / h1).
13. Refraction of Light
The bending of light rays when passing from one medium to another.
14. Laws of Refraction
- The incident ray, refracted ray, and the normal all lie in the same plane.
- The ratio of the sine of angle of incidence to the sine of angle of refraction is constant.
15. Refractive Index
Given by the ratio of the speed of light in medium 1 to the speed of light in medium 2 (n21).
16. Refraction by Spherical Lenses
Lens is a transparent medium formed by joining two pieces of spherical glass. Types:
- Convex Lens: Thicker at the center, thinner at the edges.
- Concave Lens: Thinner at the center, thicker at the edges.
17. Terms Related to a Lens
- Optical Centre: The center of the lens where light passes without deviation.
- Principal Axis: The line passing through the optical center, perpendicular to its edges.
- Principal Focus: A point on the principal axis where parallel light rays converge or appear to diverge after refraction.
18. Lens Formula
A mathematical relation connecting the object distance, image distance, and focal length.
19. Power of a Lens
The ability of a lens to converge or diverge a beam of light. SI unit: dioptre (D).
20. Power of a Combination of Lenses
The total power of multiple lenses placed together: P = P1 + P2 + P3 + …