1.0 Nature of Light
Light is a form of energy that produces the sensation of sight. It allows us to see the beautiful world around us. Without light, everything would be dark, and life as we know it would not be possible.
Sources of Light
Objects that emit light of their own are called Luminous Objects (e.g., Sun, Stars, Electric bulb). Objects that do not emit light but become visible when light falls on them are called Non-luminous Objects (e.g., Moon, Chair, Trees).
1.1 Rectilinear Propagation of Light
One of the most fundamental properties of light is that it always travels in a straight line. This property is known as the Rectilinear Propagation of Light. This is the reason why shadows are formed when an opaque object blocks the path of light.
Ray and Beam
- Ray: A very narrow path of light represented by a straight line with an arrow.
- Beam: A collection or bundle of light rays. It can be Parallel, Convergent, or Divergent.
Speed of Light
$$c = 3 \times 10^8 \, m/s$$
In vacuum or air, light travels at approximately 3,00,000 kilometres per second!
If sunlight takes about 500 seconds to reach the Earth, and the speed of light is $3 \times 10^8 \, m/s$, calculate the approximate distance between the Sun and the Earth.
Solution:
1. Given: $Speed (v) = 3 \times 10^8 \, m/s$
2. Given: $Time (t) = 500 \, s$
3. Formula: $Distance = Speed \times Time$
4. Calculation: $D = (3 \times 10^8) \times 500 = 1500 \times 10^8 \, m = 1.5 \times 10^{11} \, m$
Result: The distance is approximately 150 million kilometres.
Light does NOT require a medium (like air or water) to travel. It can travel through a vacuum. This is why we can see the light from the sun and stars through the emptiness of space.
2.0 Classification of Materials
How light interacts with an object depends entirely on the material the object is made of. Based on how much light they allow to pass through them, substances are classified into three types.
Types of Media
- Transparent: Materials that allow almost all light to pass through them. We can see through them clearly. (e.g., Clean air, pure water, clear glass).
- Translucent: Materials that allow only a small part of light to pass through them. Objects seen through them appear blurred. (e.g., Butter paper, frosted glass, muddy water).
- Opaque: Materials that do not allow any light to pass through them. We cannot see through them at all. (e.g., Wood, metal, brick wall).
2.1 Formation of Shadows
A Shadow is a region of darkness formed when an opaque object blocks the path of light. Since light travels in a straight line, it cannot "bend" around the object, leaving a dark area on the screen behind it.
Requirements for Shadow Formation
- A Source of Light.
- An Opaque Object to obstruct the light.
- A Screen (surface) on which the shadow can be formed.
Parts of a Shadow
When the light source is large (extended source), the shadow consists of two parts:
- Umbra: The innermost, completely dark region where no light reaches.
- Penumbra: The outer, partially dark region that surrounds the umbra.
Why does the size of a shadow change when you move an object closer to the light source?
Solution:
1. Light travels in straight lines (Rectilinear propagation).
2. When the object is closer to the source, it blocks a larger angle of light rays.
3. Result: The area of darkness on the screen increases, making the shadow larger.
A shadow is always black regardless of the colour of the object. This is because a shadow is simply the absence of light, not a reflection of the object's colour.
Birds flying very high in the sky do not cast a visible shadow on the ground. This is because the penumbra becomes so large and the umbra so small that it fades away before reaching the Earth!
3.0 Pinhole Camera and Eclipses
Using the principle that light travels in straight lines, we can understand two fascinating phenomena: the working of a simple Pinhole Camera and the occurrence of Eclipses in space.
3.1 The Pinhole Camera
A pinhole camera is a simple device without a lens. It consists of a light-proof box with a tiny hole (pinhole) on one side and a translucent screen (like tracing paper) on the opposite side.
Characteristics of Image
- Real: The image is formed on a screen.
- Inverted: The image is upside down due to rectilinear propagation.
- Diminished: Usually smaller than the actual object.
- Coloured: It shows the actual colours of the object.
3.2 Eclipses
An eclipse is a natural phenomenon where one heavenly body casts a shadow on another. It occurs when the Sun, Earth, and Moon align in a straight line.
1. Solar Eclipse
Occurs when the Moon comes between the Sun and the Earth. The Moon's shadow falls on certain parts of the Earth.
2. Lunar Eclipse
Occurs when the Earth comes between the Sun and the Moon. The Earth's shadow (Umbra and Penumbra) falls on the Moon.
Magnification Formula
$$m = \frac{h_i}{h_o} = \frac{v}{u}$$
Where: $h_i$ = height of image, $h_o$ = height of object, $v$ = distance of screen from pinhole, $u$ = distance of object from pinhole.
A tree 10 m tall is placed 20 m away from a pinhole camera. If the camera length is 20 cm, calculate the size of the image.
Solution:
1. Given: $h_o = 10\,m$, $u = 20\,m$, $v = 20\,cm = 0.2\,m$.
2. Formula: $\frac{h_i}{h_o} = \frac{v}{u}$
3. Calculation: $h_i = h_o \times \frac{v}{u} = 10 \times \frac{0.2}{20}$
4. $h_i = 10 \times 0.01 = 0.1\,m = 10\,cm$.
Result: The image size is $10\,cm$.
Never look at a Solar Eclipse with the naked eye. The intense light can permanently damage your retina. Use special solar filters or pinhole projection to view it safely.
4.0 Reflection of Light
We are able to see non-luminous objects because they bounce back the light that falls on them. This "bouncing back" of light from a polished or shiny surface is called Reflection. It is very similar to a ball bouncing off a wall.
4.1 Regular and Diffused Reflection
- Regular Reflection: Occurs from smooth, polished surfaces like a plane mirror. The reflected rays are parallel, producing a clear image.
- Diffused (Irregular) Reflection: Occurs from rough surfaces like a wall or paper. The rays are scattered in different directions, and no image is formed.
4.2 Terms Related to Reflection
To study reflection, we use a Ray Diagram with the following components:
- Incident Ray: The ray of light that falls on the reflecting surface.
- Reflected Ray: The ray that bounces back from the surface.
- Normal: An imaginary line perpendicular ($90^\circ$) to the surface at the point of incidence.
- Angle of Incidence ($\angle i$): Angle between the incident ray and the normal.
- Angle of Reflection ($\angle r$): Angle between the reflected ray and the normal.
Law of Reflection
$$\angle i = \angle r$$
The angle of incidence is always equal to the angle of reflection.
4.3 Characteristics of Image in a Plane Mirror
- Virtual: It cannot be caught on a screen.
- Erect: It is upright (not inverted).
- Same Size: The image is as big as the object.
- Same Distance: The image is as far behind the mirror as the object is in front of it.
- Laterally Inverted: The left side appears as the right side and vice versa.
If a ray of light strikes a plane mirror such that the angle between the incident ray and the mirror surface is $30^\circ$, what is the angle of reflection?
Solution:
1. The Normal is at $90^\circ$ to the mirror.
2. Angle of Incidence ($\angle i$) = $90^\circ - 30^\circ = 60^\circ$.
3. According to the Law of Reflection, $\angle i = \angle r$.
4. Therefore, $\angle r = 60^\circ$.
Result: The angle of reflection is $60^\circ$.
The word AMBULANCE is written laterally inverted on the front of the vehicle so that drivers ahead can read it correctly in their rearview mirrors and give way.