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Physics - Light: Reflection and Refraction

Grade 7IGCSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Light travels in straight lines called rays. When it hits a surface, it can be reflected or refracted.

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The Law of Reflection: The angle of incidence (ii) is always equal to the angle of reflection (rr). Both angles are measured from the normal, an imaginary line at 90∘90^\circ to the surface.

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Properties of images in a plane mirror: The image is virtual (cannot be projected on a screen), upright, the same size as the object, and laterally inverted (left and right are swapped).

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Refraction is the bending of light as it passes from one transparent medium to another due to a change in its speed (vv).

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When light enters a more optically dense medium (e.g., air to glass), it slows down and bends towards the normal (i>ri > r).

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When light enters a less optically dense medium (e.g., water to air), it speeds up and bends away from the normal (i<ri < r).

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The refractive index (nn) is a ratio of the speed of light in a vacuum (cc) to the speed of light in the medium (vv).

📐Formulae

∠i=∠r\angle i = \angle r

n=sin⁡isin⁡rn = \frac{\sin i}{\sin r}

n=cvn = \frac{c}{v}

speed of light in vacuum (c)≈3×108 m/s\text{speed of light in vacuum } (c) \approx 3 \times 10^8 \text{ m/s}

💡Examples

Problem 1:

A ray of light strikes a plane mirror at an angle of 35∘35^\circ to the mirror surface. Calculate the angle of reflection (rr).

Solution:

The normal is 90∘90^\circ to the surface. First, find the angle of incidence: i=90∘−35∘=55∘i = 90^\circ - 35^\circ = 55^\circ. According to the Law of Reflection, i=ri = r. Therefore, r=55∘r = 55^\circ.

Explanation:

Reflection angles are always measured from the normal, not the surface of the mirror.

Problem 2:

The speed of light in a certain type of glass is 2×108 m/s2 \times 10^8 \text{ m/s}. Given the speed of light in a vacuum is c=3×108 m/sc = 3 \times 10^8 \text{ m/s}, calculate the refractive index (nn) of the glass.

Solution:

n=cv=3×1082×108=1.5n = \frac{c}{v} = \frac{3 \times 10^8}{2 \times 10^8} = 1.5

Explanation:

The refractive index is a dimensionless number that indicates how much the medium slows down light. A higher nn means the medium is more optically dense.

Problem 3:

A ray of light travels from air into water. The angle of incidence is 45∘45^\circ and the angle of refraction is 32∘32^\circ. Calculate the refractive index of water.

Solution:

n=sin⁡(45∘)sin⁡(32∘)≈0.7070.530≈1.33n = \frac{\sin(45^\circ)}{\sin(32^\circ)} \approx \frac{0.707}{0.530} \approx 1.33

Explanation:

Snell's Law relates the angles of incidence and refraction to the refractive index of the material light is entering (when starting from air/vacuum).