ECET 2026 Physics – Reflection & Refraction

In ECET 2026 Physics, Optics is an important unit. Reflection and Refraction concepts are repeatedly asked in exams. Understanding laws, formulas, and applications helps score direct marks.

📘 Concept Notes

🔹 Reflection of Light

Definition: The process of light bouncing back into the same medium after striking a surface.
Laws of Reflection:
1. Angle of incidence $i$ = Angle of reflection $r$ .
2. Incident ray, reflected ray, and normal all lie in the same plane.
Types of Reflection:
- Regular reflection: From smooth surfaces (mirror).
- Diffuse reflection: From rough surfaces.

🔹 Refraction of Light

Definition: Bending of light when it passes from one medium to another due to change in speed.
Laws of Refraction (Snell’s Law):

$n_1 \sin i = n_2 \sin r$

Absolute Refractive Index:
$n = \dfrac{c}{v}$
where $c$ = speed of light in vacuum, $v$ = speed of light in medium.

Relative Refractive Index:

$n_{21} = \dfrac{n_2}{n_1}$

Critical Angle (C):
$\sin C = \dfrac{n_2}{n_1}$ , (when $n_1 > n_2$ )

Total Internal Reflection (TIR):
Occurs when angle of incidence > critical angle, and light reflects completely inside the medium.

⚙️ Important Formulas

Law of Reflection:

$i = r$

Snell’s Law:

$n_1 \sin i = n_2 \sin r$

Refractive Index:

$n = \dfrac{c}{v}$

Relative Refractive Index:

$n_{21} = \dfrac{n_2}{n_1}$

Critical Angle:

$\sin C = \dfrac{n_2}{n_1}$

📐 Example Problems

Example 1:
A ray of light in air enters glass ( $n = 1.5$ ) at $i = 30^\circ$ . Find angle of refraction.

Using Snell’s law:
$1 \cdot \sin 30^\circ = 1.5 \cdot \sin r$
$\sin r = \dfrac{0.5}{1.5} = 0.333$

$r = 19.5^\circ$

Example 2:
Critical angle of water ( $n = 1.33$ ) with air?

$\sin C = \dfrac{1}{1.33} = 0.75$

$C = 48.75^\circ$

🔟 10 Expected MCQs – ECET 2026

Q1. Angle of incidence = 40°, angle of reflection = ?
A) 20°
B) 40°
C) 50°
D) 60°

Q2. Snell’s law is expressed as:
A) $n_1 \cos i = n_2 \cos r$
B) $n_1 \sin i = n_2 \sin r$
C) $n_1 \tan i = n_2 \tan r$
D) None

Q3. Refractive index $n = \dfrac{c}{v}$ , if $v = \dfrac{c}{2}$ , then $n = ?$
A) 0.5
B) 1
C) 2
D) 4

Q4. Total Internal Reflection occurs when:
A) $i < C$
B) $i = C$
C) $i > C$
D) $r > C$

Q5. Critical angle is defined for light travelling:
A) From rarer to denser
B) From denser to rarer
C) From air to vacuum
D) None

Q6. If glass has $n = 1.5$ , velocity of light in glass is:
A) $3 \times 10^8 , m/s$
B) $2 \times 10^8 , m/s$
C) $4.5 \times 10^8 , m/s$
D) $1.5 \times 10^8 , m/s$