Class 7 Science Light Notes

Image formed by convex mirrors are virtual, erect and smaller in size. Can you recognise the mirrors used as side mirrors in scooters? These are convex mirrors. Convex mirrors can form images of objects spread over a large area. So, these help the drivers to see the traffic behind them.

5.6 Terms related to Spherical Mirrors

Consider a spherical mirror as shown in the given figure.

i) Centre of Curvature: The centre of the sphere of which mirror is a part is called centre of curvature. In the diagram, C is the centre of curvature.

ii) Radius of curvature: The radius of the sphere of which mirror is a part is called radius of curvature. In the diagram, PC is called radius of curvature.

iii) Pole: The centre of the reflecting surface of the mirror is called pole. In the diagram, P is called pole of the mirror.

iv) Focus: The point where all rays meet or appears to meet after reflection is called Foc In the diagram, F is called focus.

v) Focal length: The distance between Pole to the focus is called focal length. In the diagram, PF is called focal length.

5.7 Mirror Equation

The mirror equation is given by

1 + 1 = 1

1/v + 1/u = 1/f

v is called image distance

u is called object distance

f is called focal length

5.8 Lenses

Lens is an optical device used for refracting and transmitting light.

You might have seen a magnifying glass. It is used to read very small print. The magnifying glass is actually a type of lens. Lenses are widely used in spectacles, telescopes and microscopes.

Types of lenses:

i) Concave Lens: Concave lens is thinner at middle.

ii) Convex lens: Convex lens is thicker at middle.

A convex lens converges (bends inward) the light generally falling on it. Therefore, it is called a converging lens. On the other hand, a concave lens diverges (bends outward) the light and is called a diverging lens.

A convex lens forms magnified image.

A concave lens forms virtual, erect and diminished image.

5.9 Rainbow

There are seven colours in a rainbow, though it may not be easy to distinguish all of them. These are

— red, orange, yellow, green, blue, indigo and violet.

The prism splits sunlight into seven colours. The process of splitting of white light into seven colours is called dispersion.

5.10 Newton’s Disc

Take a circular cardboard disc of about 10 cm diameter. Divide this disc into seven segments. Paint the seven rainbow colours on these segments as shown in figure. 15.11. Make a small hole at the centre of the disc. Fix the disc loosely on the tip of a refill of a ball pen. Ensure that the disc rotates freely. Rotate the disc in the daylight. When the disc is rotated fast, the colours get mixed together and the disc appears to be whitish. Such a disc is popularly known as Newton’s disc.