1.0 Introduction to Light
Light is one of the most important things in our life. We can see objects around us because of light. During the day, sunlight helps us see clearly. At night, we use lamps, tube lights, torches and bulbs to see objects.
In Physics, light is a form of energy that produces the sensation of vision. This means light makes it possible for our eyes to see objects. Without light, even if an object is present in front of us, we cannot see it clearly.
Statement: Light is a form of energy that helps us see objects.
We see an object when light from the object enters our eyes. Some objects produce their own light. Some objects do not produce light, but we see them when light falls on them and reaches our eyes.
Can we see an object in complete darkness? No. If there is no light, light cannot reach our eyes from the object. That is why a book kept in a dark room cannot be seen clearly until a light source is switched on.
Objects that give out their own light are called luminous objects. The Sun, stars, candle flame, electric bulb and torch are luminous objects. They act as sources of light.
Statement: Objects that produce their own light are called luminous objects.
Objects that do not give out their own light are called non-luminous objects. A book, table, chair, Moon, wall and pencil are non-luminous objects. We can see them only when light from a luminous object falls on them and reflects into our eyes.
Statement: Objects that do not produce their own light are called non-luminous objects.
The Moon looks bright at night, but it does not produce its own light. It reflects sunlight. That is why the Moon is a non-luminous object, even though it appears bright in the night sky.
| Type of Object | Meaning | Examples |
|---|---|---|
| Luminous Object | Produces its own light | Sun, stars, candle flame, bulb |
| Non-Luminous Object | Does not produce its own light | Moon, book, chair, wall |
Sources of light may also be grouped as natural sources and artificial sources. Natural sources occur in nature, such as the Sun, stars and lightning. Artificial sources are made by humans, such as bulbs, lamps, tube lights and torches.
Do not write that the Moon is a luminous object. The Moon does not produce its own light. It reflects sunlight, so it is a non-luminous object.
Luminous = Light-giving
Non-luminous = Needs light to be seen
Light travels very fast. Sunlight takes about 8 minutes to reach Earth. This is why the Sun is our most important natural source of light and energy.
- Light is a form of energy that helps us see objects.
- Luminous objects produce their own light.
- Non-luminous objects are seen when light falls on them and reaches our eyes.
Why can we not see a book clearly in complete darkness?
2.0 Rectilinear Propagation of Light
Light usually travels in a straight line. This property of light is called rectilinear propagation of light. The word "rectilinear" means straight-line motion. This is why a beam of sunlight entering through a small hole appears as a straight path.
Statement: The property of light to travel in a straight line is called rectilinear propagation of light.
A very thin path of light is called a ray of light. A group of light rays is called a beam of light. In diagrams, a ray of light is shown using a straight line with an arrow. The arrow shows the direction in which light travels.
Statement: A ray of light is a narrow path of light. A beam of light is a group of light rays travelling together.
How do we know that light travels in a straight line? Take three cardboard sheets with small holes. Keep them in a straight line and place a candle at one end. You can see the flame through the holes only when all holes are exactly aligned. If one sheet is moved slightly, the flame cannot be seen. This shows that light travels in a straight line.
The straight-line travel of light also explains the formation of shadows. When an opaque object blocks light, light cannot pass through it. So, a dark region is formed behind the object. This dark region is called a shadow.
When sunlight enters a dusty room through a small gap, we can sometimes see a straight beam of light. Car headlights, torchlight and projector light also show that light travels in straight paths.
Objects can be grouped according to how they allow light to pass through them. The three main groups are transparent, translucent and opaque objects.
| Type of Object | Meaning | Can We See Clearly Through It? | Examples |
|---|---|---|---|
| Transparent | Allows most light to pass through | Yes | Clear glass, clean water, air |
| Translucent | Allows some light to pass through | No, objects look blurred | Butter paper, frosted glass |
| Opaque | Does not allow light to pass through | No | Wood, metal, cardboard, stone |
Do not confuse translucent and transparent objects. Transparent objects allow clear vision through them. Translucent objects allow only partial light, so objects behind them look unclear or blurred.
T-T-O helps you remember:
Transparent = Total clear view
Translucent = Tiny/partial light
Opaque = Obstructs light
Light travels in a straight line in the same medium, such as air. When light enters a different medium like water or glass, its path can bend. This bending is called refraction, which is studied in higher classes.
- Light travels in a straight line in the same medium.
- A ray is a narrow path of light, and a beam is a group of rays.
- Objects may be transparent, translucent or opaque depending on how much light they allow to pass.
Why does an opaque object form a shadow when light falls on it?
3.0 Shadows and Eclipses
A shadow is formed when an opaque object blocks the path of light. Since light travels in a straight line, it cannot bend around an ordinary opaque object easily. So, a dark region is formed behind the object. This dark region is called a shadow.
Statement: A shadow is a dark region formed behind an opaque object when it blocks the path of light.
Three things are needed to form a shadow: a source of light, an opaque object and a screen or surface on which the shadow is formed. For example, if you place your hand in front of a torch and keep a wall behind it, the shadow of your hand appears on the wall.
Why is a shadow dark? The opaque object blocks light from reaching some part of the screen. The part where light does not reach appears dark. This dark part is the shadow.
A shadow gives only the outline or shape of the object. It does not show the colour or details of the object. For example, the shadow of a red ball and a blue ball may look the same because shadows are dark regions, not coloured images.
| Condition | Role in Shadow Formation | Example |
|---|---|---|
| Source of Light | Provides light rays | Sun, torch, bulb |
| Opaque Object | Blocks light | Hand, book, ball |
| Screen | Surface where shadow is seen | Wall, floor, paper |
The size of a shadow can change. If the object is moved closer to the light source, the shadow usually becomes bigger. If the object is moved farther from the light source and closer to the screen, the shadow usually becomes smaller.
During morning and evening, our shadows look long because sunlight reaches us at a slanting angle. Around noon, the Sun is higher in the sky, so shadows become shorter.
Sometimes a shadow has a very dark central region and a lighter outer region. The very dark part is called the umbra. The lighter shadow region around it is called the penumbra. At Class 6 level, remember that umbra is the complete shadow and penumbra is the partial shadow.
Statement: Umbra is the completely dark part of a shadow, while penumbra is the partially dark region around the umbra.
An eclipse is a natural shadow event in space. It happens when the Sun, Earth and Moon come in a straight line. Since light travels in straight lines, one object can block sunlight and cast a shadow on another object.
A solar eclipse occurs when the Moon comes between the Sun and the Earth. The Moon blocks sunlight and casts its shadow on Earth. A lunar eclipse occurs when the Earth comes between the Sun and the Moon. Earth blocks sunlight from reaching the Moon.
Do not confuse solar eclipse and lunar eclipse. In a solar eclipse, the Moon comes between the Sun and Earth. In a lunar eclipse, Earth comes between the Sun and Moon.
Solar Eclipse: Moon blocks Sunlight.
Lunar Eclipse: Earth blocks sunlight from the Moon.
Never look directly at the Sun during a solar eclipse without proper safety filters. Sunlight can harm the eyes.
- A shadow forms when an opaque object blocks light.
- Shadow formation needs a light source, an opaque object and a screen.
- Eclipses occur due to shadows formed by the Sun, Earth and Moon coming in a straight line.
Why does the size of your shadow change during different times of the day?
4.0 Reflection of Light
When light falls on a smooth and shiny surface, it bounces back. This bouncing back of light is called reflection of light. We can see ourselves in a mirror because light from our face falls on the mirror and reflects back to our eyes.
Statement: Reflection is the bouncing back of light from a surface after it strikes that surface.
A smooth and polished surface reflects light better than a rough surface. A plane mirror, still water and polished metal can reflect light clearly. A wall, paper or cloth also reflects light, but not in a clear and regular way.
Why can we see our face clearly in a mirror but not on a wall? A mirror has a smooth surface, so light rays reflect in an orderly way and form a clear image. A wall has a rough surface, so light rays scatter in different directions and do not form a clear image.
A plane mirror is a flat mirror. It forms an image of objects placed in front of it. The image formed by a plane mirror has some important properties that are often asked in exams.
Statement: A plane mirror is a flat reflecting surface that forms an image of an object placed in front of it.
| Property | Meaning | Example |
|---|---|---|
| Erect Image | Image is upright, not upside down | Your face appears upright in a mirror |
| Same Size | Image size is equal to object size | A mirror does not make you actually taller |
| Laterally Inverted | Left and right sides appear interchanged | Your right hand appears as left hand in the mirror |
| Virtual Image | Image appears behind the mirror and cannot be caught on a screen | Image in a dressing mirror |
Mirrors are used in homes, vehicles, barber shops, dressing rooms and periscopes. Rear-view mirrors in vehicles help drivers see traffic behind them. Mirrors are also used in shops and security systems to view wider areas.
Reflection can be of two simple types: regular reflection and irregular reflection. Regular reflection takes place from a smooth surface, such as a mirror. Irregular reflection takes place from a rough surface, such as a wall or paper.
| Type of Reflection | Surface | Result | Example |
|---|---|---|---|
| Regular Reflection | Smooth and polished surface | Clear image is formed | Plane mirror |
| Irregular Reflection | Rough surface | Light scatters in many directions | Wall, paper, cloth |
Do not write that a plane mirror image is exactly the same in all ways. It is upright and of the same size, but it is laterally inverted. This means left and right appear interchanged.
Mirror Image = ESSV
E = Erect
S = Same size
S = Same distance behind mirror as object is in front
V = Virtual
The word "AMBULANCE" is written in reverse on the front of an ambulance so that drivers can read it correctly in their rear-view mirrors.
- Reflection is the bouncing back of light from a surface.
- A plane mirror forms an erect, virtual and laterally inverted image.
- Smooth surfaces give regular reflection, while rough surfaces give irregular reflection.
Why can a mirror form a clear image, but a rough wall cannot?
5.0 Applications, Revision and Exam Readiness
Light is essential for life and vision. It helps us see objects, read books, move safely, grow plants, use mirrors and operate many modern devices. Without light, the world would be dark, and we would not be able to see objects around us clearly.
Light is used in homes, schools, hospitals, vehicles, traffic signals, cameras, projectors, street lights and solar panels. Sunlight helps plants prepare food. Mirrors help us see reflected images. Torches and headlights help us see in darkness.
Why is light needed for seeing? We see an object only when light from the object enters our eyes. Luminous objects send their own light. Non-luminous objects are seen when light falls on them and reflects into our eyes.
Do not write that we see all objects because they produce light. Only luminous objects produce their own light. Most objects around us are non-luminous and are seen because they reflect light.
| Topic | Quick Revision Point | Example |
|---|---|---|
| Light | A form of energy that helps us see objects | Sunlight, bulb light |
| Luminous Object | Produces its own light | Sun, stars, candle flame |
| Non-Luminous Object | Does not produce its own light | Moon, book, chair |
| Rectilinear Propagation | Light travels in a straight line in the same medium | Torch beam, sunlight through a hole |
| Shadow | Dark region formed when an opaque object blocks light | Shadow of a hand on wall |
| Reflection | Bouncing back of light from a surface | Image in a mirror |
| Plane Mirror Image | Erect, same size, virtual and laterally inverted | Dressing mirror image |
No major numerical formula in this chapter
- Key Rule 1: Light travels in a straight line in the same medium.
- Key Rule 2: Shadow forms when an opaque object blocks light.
- Key Rule 3: Reflection is the bouncing back of light.
- Key Rule 4: Plane mirror image is erect, virtual, same size and laterally inverted.
Light Revision = L-S-R-M
L = Luminous objects give light
S = Straight line travel
R = Reflection from mirrors
M = Mirror image is laterally inverted
- Define light.
- Differentiate between luminous and non-luminous objects.
- What is rectilinear propagation of light?
- What are transparent, translucent and opaque objects?
- State three conditions required for shadow formation.
- What is reflection of light?
- Write two properties of the image formed by a plane mirror.
- Light helps us see objects and usually travels in a straight line.
- Shadows form when opaque objects block light.
- Reflection from smooth surfaces like mirrors forms clear images.
Why is light essential for seeing objects around us?