Science: Class 7 — Chapter 11: Light

11.1 Light Travels Along a Straight Line

Light always travels along straight lines.
This can be demonstrated by attempting to view a candle flame through a straight pipe versus a bent pipe; the flame is only visible through the straight pipe.

11.2 Reflection of Light

Reflection of light is the change in direction of light when it falls on a shiny or polished surface (like a mirror, stainless steel plate, or water).
An object is the source of light or body placed in front of the mirror, while the image is the likeness formed by reflection.

The image is erect (upright) and of the same size as the object.
The image is formed at the same distance behind the mirror as the object is in front of it.
The image is a virtual image because it cannot be obtained on a screen.

11.3 Right or Left!

In a plane mirror, the left side of the object appears on the right side of the image, and the right side of the object appears on the left side of the image.
Ambulance Writing: The word "AMBULANCE" is written in reverse on the front of the vehicle so that drivers ahead can read it correctly in their rear-view mirrors.

11.4 Playing with Spherical Mirrors

A spherical mirror is a mirror with a curved reflecting surface, which can be imagined as a part of a cut rubber ball.
Concave mirror: The reflecting surface is curved inwards (like the inner side of a spoon).
Convex mirror: The reflecting surface is curved outwards (like the outer side of a spoon).

Real image: An image that can be formed and obtained on a screen.
Virtual image: An image that cannot be obtained on a screen.
Concave Mirror Images: Can form both real and virtual images. The image can be smaller, larger, or of the same size as the object, and can be erect or inverted depending on the distance.
Convex Mirror Images: Always forms a virtual, erect, and smaller image than the object.

Concave mirrors are used by dentists to see enlarged images of teeth, by doctors to examine eyes, ears, nose, and throat, and as reflectors in torches and car headlights.
Convex mirrors are used as side/rear-view mirrors in vehicles because they can form images of objects spread over a large area, helping drivers see traffic behind them.

11.5 Images Formed by Lenses

Lenses are transparent materials through which light can pass, widely used in spectacles, telescopes, and microscopes.
Convex lens: Thicker in the middle than at the edges. It acts as a converging lens because it bends light rays inwards.
Concave lens: Thinner in the middle than at the edges. It acts as a diverging lens because it bends light rays outwards.

Convex Lens Images: Can form real and inverted images. However, when the object is placed very close to the lens, it forms a virtual, erect, and magnified image. This is why it is used as a magnifying glass.
Concave Lens Images: Always forms a virtual, erect, and smaller image than the object.

11.6 Sunlight — White or Coloured?

Sunlight, often called white light, is actually a mixture of seven colors: Red, Orange, Yellow, Green, Blue, Indigo, and Violet.
A prism can split a beam of sunlight into its constituent seven colors.
This phenomenon is also seen in rainbows, soap bubbles, and light reflected from the surface of a Compact Disk (CD).
Newton's disc: A circular cardboard disc painted with the seven rainbow colors appears whitish when rotated rapidly, demonstrating that white light is composed of these seven colors.