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Light Energy - Uses of Mirrors

Grade 7ICSE

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

🔑Concepts

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Reflection of Light: The process of sending back the light rays which fall on the surface of an object. The Laws of Reflection state: 1. The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane. 2. The angle of incidence ∠i\angle i is always equal to the angle of reflection ∠r\angle r.

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Plane Mirror Properties: Images formed are virtual, erect, the same size as the object, and at the same distance behind the mirror as the object is in front. It also exhibits lateral inversion (left appears right and vice versa).

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Uses of Plane Mirrors: Used as looking glasses, in periscopes (two mirrors fixed at 45∘45^\circ to the frame), and in kaleidoscopes (three mirrors at 60∘60^\circ to each other).

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Concave Mirror (Converging Mirror): Curving inwards, it can form both real and virtual images. If the object is very close, it forms a magnified, erect, and virtual image.

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Uses of Concave Mirrors: Shaving mirrors or makeup mirrors (to see an enlarged view), used by dentists to see enlarged images of teeth, and in car headlights or searchlights to produce a parallel beam of light.

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Convex Mirror (Diverging Mirror): Curving outwards, it always forms a virtual, erect, and diminished (smaller) image regardless of the object's distance.

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Uses of Convex Mirrors: Rear-view mirrors in vehicles because they provide a wider field of view to see traffic behind, and as security mirrors in shops and parking lots.

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Relationship between Focal Length and Radius: For spherical mirrors, the focal length ff is half the radius of curvature RR (f=R2f = \frac{R}{2}).

📐Formulae

∠i=∠r\angle i = \angle r

f=R2f = \frac{R}{2}

n=(360∘θ)−1n = \left( \frac{360^{\circ}}{\theta} \right) - 1

💡Examples

Problem 1:

If two plane mirrors are placed at an angle of 90∘90^{\circ} to each other, how many images of an object placed between them will be formed?

Solution:

n=360∘90∘−1=4−1=3n = \frac{360^{\circ}}{90^{\circ}} - 1 = 4 - 1 = 3

Explanation:

Using the formula for the number of images n=360∘θ−1n = \frac{360^{\circ}}{\theta} - 1, where θ\theta is the angle between the mirrors. Here θ=90∘\theta = 90^{\circ} results in 33 images.

Problem 2:

A concave mirror has a radius of curvature of 40 cm40\text{ cm}. Calculate its focal length ff.

Solution:

f=R2=40 cm2=20 cmf = \frac{R}{2} = \frac{40\text{ cm}}{2} = 20\text{ cm}

Explanation:

The focal length of a spherical mirror is defined as half of its radius of curvature.

Problem 3:

Why is a convex mirror used as a rear-view mirror in a car instead of a plane mirror?

Solution:

A convex mirror forms a diminished image, which allows for a much wider field of view.

Explanation:

While a plane mirror gives an image of the same size, a convex mirror (divergingdiverging) covers a larger area behind the driver, enabling them to see more traffic than a plane mirror of the same size would allow.