Light – Reflection & Refraction
Class 10 (CBSE Science)
Learning Objectives
• Understand the behaviour of light when it encounters a surface.
• Memorise and apply the laws of reflection and refraction.
• Know how to calculate angles, refractive indices and critical angles.
• Relate the concepts to everyday optical devices.
1. Simple Explanation
| Phenomenon | What Happens? | Key Idea |
|---|---|---|
| Reflection | Light bounces back from a surface. | The angle at which the light arrives (incidence) equals the angle at which it leaves (reflection). |
| Refraction | Light bends while passing from one medium to another. | The speed of light changes → the direction changes. The amount of bending is governed by the refractive indices of the two media. |
2. Key Points
2.1 Reflection
| Concept | Details |
|---|---|
| Law of Reflection | (\displaystyle \theta_{\text{i}} = \theta_{\text{r}}) (incidence = reflection). |
| Plane Mirror | Image is real (virtual in the case of plane mirror?) Actually plane mirror produces a virtual image. |
| Concave Mirror | Image can be real or virtual depending on the object distance (u). |
| Convex Mirror | Image is always virtual, upright and reduced. |
| Angle of Incidence | Angle between incident ray and normal. |
| Normal | Perpendicular line to the surface at the point of incidence. |
| Incident Ray | Ray before hitting the surface. |
| Reflected Ray | Ray after reflecting. |
Mirror Formula (concave & convex)
[
\frac{1}{f} = \frac{1}{u} + \frac{1}{v}
]
where
(f) = focal length, (u) = object distance, (v) = image distance (positive if real, negative if virtual).
Magnification
[
m = \frac{v}{u} = -\frac{h_i}{h_o}
]
(h_i) = image height, (h_o) = object height.
2.2 Refraction
| Concept | Details |
|---|---|
| Law of Refraction (Snell’s Law) | (\displaystyle n_1 \sin\theta_1 = n_2 \sin\theta_2) |
| Refractive Index | (n = \frac{c}{v}) where (c) = speed of light in vacuum, (v) = speed in medium. |
| Critical Angle | (\displaystyle \theta_c = \sin^{-1}!\left(\frac{n_2}{n_1}\right)) (only defined when (n_1>n_2)). |
| Total Internal Reflection | When (\theta_1 > \theta_c), light does not refract but reflects entirely inside the denser medium. |
| Prisms | Split white light into spectrum due to dispersion (different (n) for different wavelengths). |
| Applications | Lenses, fiber optics, optical instruments, cameras, eye glasses. |
Important Refractive Indices (approx.)
| Medium | (n) |
|---|---|
| Air | 1.00 |
| Water | 1.33 |
| Glass (common) | 1.50 |
| Diamond | 2.42 |
3. Worked Example
Problem:
A light ray travels from water into glass.
Given:
- Angle of incidence in water, (\theta_1 = 30^\circ).
- Refractive indices: (n_{\text{water}} = 1.33), (n_{\text{glass}} = 1.50).
Find:
- Angle of refraction (\theta_2).
- Whether total internal reflection occurs.
Step‑by‑Step
-
Apply Snell’s Law
[ n_1 \sin\theta_1 = n_2 \sin\theta_2 ] [ 1.33 \sin 30^\circ = 1.50 \sin\theta_2 ] [ 1.33 \times 0.5 = 1.50 \sin\theta_2 ] [ 0.665 = 1.50 \sin\theta_2 ] [ \sin\theta_2 = \frac{0.665}{1.50} = 0.4433 ] -
Compute (\theta_2)
[ \theta_2 = \sin^{-1}(0.4433) \approx 26.4^\circ ] -
Check for Total Internal Reflection
Total internal reflection can only happen when light goes from denser to rarer medium.
Here, light moves from water ((n=1.33)) to glass ((n=1.50)) → no TIR.
Answer
- (\theta_2 \approx 26.4^\circ).
- No total internal reflection.
4. Quick Revision Summary
| Topic | Key Take‑Away |
|---|---|
| Reflection | (\theta_i = \theta_r). Mirror formulas give image |