Light — Class 8 Science

Light is a form of energy that helps us see objects around us. Without light, we cannot see anything.

Key Properties of Light:

• Light travels in straight lines (rectilinear propagation)
• Speed of light = 3,00,000 km/second (fastest thing in the universe!)
• Light can travel through vacuum (empty space) - unlike sound
• Light does not need a medium to travel

Sources of Light:

• Natural sources: Sun, stars, fireflies
• Artificial sources: Bulb, tube light, candle

When light falls on a surface, it bounces back. This bouncing back of light is called reflection.

Important Terms:

• Incident Ray: The light ray that falls on the surface
• Reflected Ray: The light ray that bounces back from the surface
• Normal: An imaginary line perpendicular (90°) to the surface at the point where light hits
• Point of Incidence: The point where the incident ray hits the surface
• Angle of Incidence (∠i): Angle between incident ray and normal
• Angle of Reflection (∠r): Angle between reflected ray and normal

Laws of Reflection:
1. Angle of incidence = Angle of reflection (∠i = ∠r)
2. Incident ray, reflected ray, and normal all lie in the same plane

There are two types of reflection based on the surface:

1. Regular Reflection:

• Occurs on smooth, shiny surfaces (mirror, still water)
• Parallel rays remain parallel after reflection
• Forms clear images

2. Irregular (Diffused) Reflection:

• Occurs on rough, uneven surfaces (wall, paper, wood)
• Parallel rays scatter in different directions
• Does not form clear images
• This is why we can see objects from all directions!

A plane mirror is a flat, smooth reflecting surface. It is the most common type of mirror we use daily.

Properties of Image in Plane Mirror:

• Image is virtual (cannot be obtained on a screen)
• Image is erect (upright, not upside down)
• Image is of same size as the object
• Image is at same distance behind mirror as object is in front
• Image is laterally inverted (left-right reversed)

Lateral Inversion:

When you look in a mirror, your left hand appears as right hand and vice versa. This left-right reversal is called lateral inversion.

Example: The word "AMBULANCE" is written reversed on ambulances so it appears correct in the rear-view mirror of vehicles ahead.

Spherical mirrors are mirrors whose reflecting surface is a part of a sphere. Before studying them, learn these terms:

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

A concave mirror has its reflecting surface curving inward, like the inside of a spoon.

Properties:

• Reflecting surface curves inward (like a cave - conCAVE)
• Also called converging mirror because parallel rays converge (meet) at focus
• Can form both real and virtual images
• Image can be enlarged, diminished, or same size depending on object position

Uses of Concave Mirror:

• Dentists - to see enlarged images of teeth
• Torches & Headlights - to produce parallel beams of light
• Solar Cookers - to focus sunlight and generate heat
• Shaving/Makeup Mirrors - to see enlarged face

A convex mirror has its reflecting surface curving outward, like the back of a spoon.

Properties:

• Reflecting surface curves outward (bulges out)
• Also called diverging mirror because parallel rays diverge (spread out)
• Always forms virtual, erect, and diminished images
• Has a wider field of view than plane mirrors

Uses of Convex Mirror:

• Rear-view mirrors in vehicles - wider field of view to see more traffic
• Blind turns on roads - to see oncoming traffic around corners
• Security mirrors in shops - to watch larger areas
• ATM machines - to show surroundings for security

A lens is a transparent material (usually glass) with curved surfaces that bends light rays.

Convex Lens (Converging Lens):

• Thicker in the middle, thinner at edges
• Converges (brings together) parallel light rays
• Can form both real and virtual images
• Uses: Magnifying glass, camera, microscope, spectacles for hypermetropia (long-sightedness)

Concave Lens (Diverging Lens):

• Thinner in the middle, thicker at edges
• Diverges (spreads out) parallel light rays
• Always forms virtual, erect, and diminished images
• Uses: Spectacles for myopia (short-sightedness)

Dispersion is the splitting of white light into its seven component colours when it passes through a prism.

The Visible Spectrum (VIBGYOR)

Violet - Indigo - Blue - Green - Yellow - Orange - Red

Important Points:

• Violet bends the MOST (shortest wavelength)
• Red bends the LEAST (longest wavelength)
• Sir Isaac Newton first demonstrated this experiment

A rainbow is a natural spectrum formed by dispersion of sunlight by water droplets in the atmosphere.

How Rainbow Forms:

1. Sunlight enters tiny water droplets in the air
2. Light gets refracted (bent) when entering the droplet
3. Light gets dispersed into seven colours inside the droplet
4. Light reflects from the back surface of the droplet
5. Light refracts again when leaving the droplet
6. We see the rainbow!

Key Points about Rainbow:

• Rainbow always appears opposite to the Sun
• Water droplets act like tiny prisms
• In a rainbow: Red is on the outer edge, Violet is on the inner edge
• Rainbow is always seen when Sun is behind you and rain is in front

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

Easy way to remember:

• Real image = Really there (can catch on screen)
• Virtual image = Not really there (only appears to be there)