Light — Class 7 Science

Light is a form of energy that makes things visible to us. We can see objects only when light from them reaches our eyes. Objects that give out their own light are called luminous objects (e.g. the Sun, a burning candle, a torch). Objects that do not give out their own light are called non-luminous objects (e.g. the Moon, a book, a table). Non-luminous objects are visible because they reflect light falling on them.

Light travels at a very high speed of about 3,00,000 km per second (3 x 10⁸ m/s). It can travel through vacuum (empty space), which is how sunlight reaches the Earth.

Light always travels in straight lines. This property is called rectilinear propagation of light. The path along which light travels is called a ray of light, and a group of parallel rays is called a beam of light.

Evidence that light travels in straight lines:

• Shadows: When an opaque object is placed in the path of light, a dark shadow forms behind it. This happens because light cannot bend around the object.
• Pinhole Camera: A pinhole camera forms an inverted image because light rays travel in straight lines through the tiny hole.
• Eclipses: Solar and lunar eclipses occur because light from the Sun travels in straight lines and gets blocked by the Moon or Earth.

When a ray of light falls on a smooth, shiny surface (like a mirror), it bounces back. This bouncing back of light is called reflection of light.

Key terms in reflection:

• Incident ray: The ray of light that falls on the mirror surface.
• Reflected ray: The ray of light that bounces back from the mirror surface.
• Point of incidence: The point where the incident ray strikes the mirror.
• Normal: An imaginary line drawn perpendicular (at 90 degrees) to the mirror surface at the point of incidence.
• Angle of incidence (i): The angle between the incident ray and the normal.
• Angle of reflection (r): The angle between the reflected ray and the normal.

There are two laws of reflection that apply to all types of reflecting surfaces:

First Law: The angle of incidence is always equal to the angle of reflection.

Angle of incidence (i) = Angle of reflection (r)

Second Law: The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.

These laws apply to all types of reflection, whether on a smooth mirror or a rough surface.

Reflection can be of two types depending on the nature of the reflecting surface:

1. Regular Reflection:

• Occurs on smooth, polished surfaces like mirrors, still water, or polished metals.
• When parallel rays of light fall on a smooth surface, they are reflected as parallel rays in a definite direction.
• This produces a clear image.

2. Diffused (Irregular) Reflection:

• Occurs on rough, uneven surfaces like walls, paper, wood, or unpolished metal.
• When parallel rays of light fall on a rough surface, they are reflected in different directions because the normals at different points are not parallel.
• This does not produce a clear image, but it is the reason we can see objects from different angles.

A plane mirror is a flat, smooth reflecting surface, usually made of glass with a thin coating of silver or aluminium on one side.

Properties of the image formed by a plane mirror:

• The image is virtual (it cannot be obtained on a screen).
• The image is erect (upright, not upside down).
• The image is 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 laterally inverted (left and right are interchanged).

Lateral inversion is the phenomenon where the left side of an object appears as the right side in the image, and the right side appears as the left side. This is why the word AMBULANCE is written in reverse on the front of ambulances -- so that it appears correct when seen in the rear-view mirror of vehicles ahead.

Examples of lateral inversion:

• If you raise your right hand, your image in the mirror raises the left hand.
• The letter 'b' appears as 'd' in the mirror.
• The letter 'p' appears as 'q' in the mirror.
• Some letters and numbers look the same in the mirror (like A, H, I, M, O, T, U, V, W, X, Y and 0, 8).

Spherical mirrors are mirrors whose reflecting surfaces form a part of a sphere. There are two types of spherical mirrors:

Important terms related to spherical mirrors:

• Pole (P): The centre of the reflecting surface of the mirror.
• Centre of Curvature (C): The centre of the sphere of which the mirror is a part.
• Principal Axis: The straight line passing through the pole (P) and the centre of curvature (C).
• Radius of Curvature (R): The radius of the sphere of which the mirror forms a part. It is the distance from P to C.
• Focus (F) / Principal Focus: The point on the principal axis where parallel rays of light meet (in concave) or appear to diverge from (in convex) after reflection.
• Focal Length (f): The distance between the pole (P) and the focus (F).

Focal Length (f) = Radius of Curvature (R) / 2

A concave mirror has its reflecting surface curved inward, like the inside of a spoon. It is also called a converging mirror because it converges (brings together) parallel rays of light to a single point -- the focus (F).

Properties of a concave mirror:

• The reflecting surface curves inward (like a cave).
• Parallel rays of light converge at the focus after reflection.
• It can form both real and virtual images depending on the position of the object.
• The image can be enlarged, diminished, or the same size as the object.

Uses of concave mirrors:

• Dentists use concave mirrors to see enlarged images of teeth.
• Torches and headlights use concave mirrors to produce a strong, parallel beam of light.
• Shaving mirrors are concave mirrors that produce an enlarged image of the face.
• Solar cookers use large concave mirrors to focus sunlight and generate heat for cooking.

A convex mirror has its reflecting surface curved outward. It is also called a diverging mirror because it spreads out (diverges) parallel rays of light after reflection. The reflected rays appear to come from a point behind the mirror (the virtual focus).

Properties of a convex mirror:

• The reflecting surface curves outward (bulges out).
• Parallel rays diverge after reflection and appear to come from the focus behind the mirror.
• It always forms a virtual, erect, and diminished (smaller) image.
• It has a wider field of view compared to a plane mirror of the same size.

Uses of convex mirrors:

• Rear-view mirrors in vehicles -- they give a wider field of view so the driver can see more of the road behind.
• Blind turns on roads -- convex mirrors are placed at sharp curves to show vehicles coming from the other side, helping to prevent accidents.
• Security mirrors in shops and ATMs -- to give a wide-angle view of the area.

A convex lens is thicker in the middle and thinner at the edges. It is also called a converging lens because it converges (brings together) parallel rays of light passing through it to a point called the focus.

Properties of a convex lens:

• Thicker at the centre, thinner at the edges.
• Converges parallel rays of light to the focus.
• Can form both real and virtual images depending on the position of the object.
• When the object is very close (within the focal length), it acts as a magnifying glass and forms an enlarged, virtual, erect image.

Uses of convex lens:

• Magnifying glass -- to see small objects enlarged.
• Spectacles for hypermetropia (long-sightedness) -- helps focus images on the retina.
• Camera lenses -- to focus light and form images on film or sensor.
• Microscopes and telescopes -- use combinations of convex lenses.

A concave lens is thinner in the middle and thicker at the edges. It is also called a diverging lens because it spreads out (diverges) parallel rays of light passing through it. The diverged rays appear to come from a point on the same side as the incoming light -- this is the virtual focus.

Properties of a concave lens:

• Thinner at the centre, thicker at the edges.
• Diverges parallel rays of light.
• Always forms a virtual, erect, and diminished (smaller) image.
• The image is formed on the same side as the object.

Uses of concave lens:

• Spectacles for myopia (short-sightedness or nearsightedness) -- helps diverge light so it focuses correctly on the retina.
• Used in peepholes of doors -- gives a wider view of who is outside.

Sunlight or white light is not a single colour. It is actually a mixture of seven colours. When white light passes through a glass prism, it splits into its seven constituent colours. This splitting of white light into seven colours is called dispersion of light.

The band of seven colours obtained after dispersion is called a spectrum. The seven colours of the spectrum in order are:

V - I - B - G - Y - O - R
Violet - Indigo - Blue - Green - Yellow - Orange - Red

Key points about dispersion:

• Violet light bends the most (has the shortest wavelength).
• Red light bends the least (has the longest wavelength).
• The recombination of all seven colours gives back white light. This was first demonstrated by Isaac Newton.

A Newton's disc (also called a colour disc) is a disc divided into seven sectors, each painted with one of the seven colours of the spectrum (VIBGYOR). When this disc is rotated rapidly, the seven colours merge and the disc appears to be white.

This experiment proves that white light is actually a combination of seven colours. When the disc spins fast, our eyes cannot distinguish the individual colours, and they blend together to form white.

Newton's disc demonstrates the reverse of dispersion -- while a prism splits white light into seven colours, a rapidly spinning Newton's disc combines the seven colours back into white.

A rainbow is a natural spectrum that appears in the sky when sunlight passes through tiny water droplets present in the atmosphere after rain.

How a rainbow forms:

• Sunlight enters a water droplet and gets refracted (bent) as it enters.
• Inside the droplet, the light gets dispersed into its seven constituent colours.
• The dispersed light then gets reflected from the inner surface of the droplet.
• The light is refracted again as it exits the droplet.
• Each water droplet acts like a tiny prism, splitting white sunlight into the spectrum.

Important facts about rainbows:

• A rainbow always appears opposite to the Sun (the Sun must be behind the observer).
• In a rainbow, red is on the outer edge and violet is on the inner edge.
• A rainbow is formed by the combined effects of refraction, dispersion, and internal reflection of light inside water droplets.
• Rainbows can also be seen near waterfalls and garden sprinklers when conditions are right.

Images formed by mirrors and lenses can be classified as real or virtual: