Light and sources of light
When the power goes off at night, you cannot see the room, even though your eyes are open. When someone lights a candle, everything becomes visible at once. We need both eyes and light to see.
Objects that produce their own light are sources of light (प्रकाशस्रोत). Light spreads from them in all directions. Sources are natural (Sun, stars, lightning, fireflies) or man-made (oil lamp, torch, lantern, electric bulb).
Rule 1: Natural and man-made sources — The Sun is the largest source of light for the Earth. Sunlight lets plants prepare food.
- Easy : Natural: Sun, stars, lightning, fireflies.
- Medium : Man-made: oil lamp, torch, lantern, electric bulb.
- Exam-type : The Moon looks bright but is not a source; it reflects sunlight.
Rule 2: Seeing light — We see objects when light from them reaches our eyes. We see dust along a beam, but never light itself.
- Easy : In a dark room a torch beam shows only the dust in it.
- Medium : A glowing bulb is seen because its own light reaches the eye.
- Exam-type : A book is seen because light reflected from it reaches the eye.
A source of light makes its own light. A book, a mirror and the Moon only reflect light.
Calling the Moon a source of light. It shines by reflecting sunlight.
Memory clue: Source = Self-made light.
Spot the difference
| Type | Example |
|---|---|
| Natural source | Sun, stars, firefly |
| Man-made source | Torch, lantern |
| Not a source | Moon, mirror, book |
Practice questions
- 1What do we need to see an object?
- 2What is a source of light?
- 3Name two natural sources of light.
- 4Name two man-made sources of light.
- 5Which is the largest source of light for the Earth?
- 6Why can we see dust in a torch beam but not the light?
- 7Is the Moon a source of light? Explain.
- 8Why do we see a book in a lighted room?
Show answers (try them yourself first)
- 1Eyes and light.
- 2An object that produces its own light.
- 3Sun and fireflies.
- 4Torch and electric bulb.
- 5The Sun
- 6Light is seen only when it reaches the eye; dust reflects light into our eyes.
- 7No. It reflects sunlight; it is not self-luminous.
- 8Light from the lamp falls on the book and is reflected to our eyes.
Linear propagation of light
You cannot see around a corner. When a bright torch is on, the beam makes a straight line, never a curve. Light moves in straight lines.
Light travels in a straight line. This is the linear propagation of light (प्रकाशाचा सरळ रेषेतील प्रवास). Sound needs a medium (solid, liquid or gas) to travel, but light needs no medium.
In the figure : In the figure the holes of the three cardboards are in a straight line, so the eye can see the candle flame through them.
Rule 1: Proof of straight travel — The candle is seen through three cardboards only when the holes are in a straight line.
- Easy : Moving one cardboard sideways hides the flame.
- Medium : You cannot see through a bent pipe, but you can through a straight one.
- Exam-type : Sunlight coming through a gap in a window makes a straight beam.
Rule 2: Light does not need a medium — Air extends only about 100 km above the Earth, yet sunlight reaches us from the Sun, 15 crore km away.
- Easy : Sound cannot travel through empty space, but light can.
- Medium : Sunlight crosses empty space to reach the Earth.
- Exam-type : Light also travels through air, water and glass.
Light travels in straight lines. It needs no medium.
Thinking light bends around corners. It travels straight; this is why a shadow has the shape of the object.
Memory clue: Light = Line; Sound = Substance (needs a medium).
Spot the difference
| Point | Light | Sound |
|---|---|---|
| Path | Straight line | Spreads in a medium |
| Medium needed? | No | Yes |
| In space | Travels | Does not travel |
Practice questions
- 1How does light travel?
- 2What is a medium?
- 3Does light need a medium?
- 4How far is the Sun from the Earth?
- 5How far above the Earth does air extend?
- 6In the three-cardboard experiment, what happens if one cardboard is moved?
- 7Why can we not see through a bent pipe?
- 8Meena says sunlight reaches us through air all the way from the Sun. Correct her.
Show answers (try them yourself first)
- 1In a straight line.
- 2A solid, liquid or gas through which sound travels.
- 3No
- 4About 15 crore km
- 5About 100 km
- 6The flame is no longer seen.
- 7Light travels in a straight line and cannot bend around the pipe.
- 8Air extends only about 100 km; the rest is empty space, and light crosses it without a medium.
Pinhole camera
On a sunny day, light coming through a tiny hole in a closed window makes a small upside-down picture of the outside tree on the opposite wall. This is how a pinhole camera works, without any lens.
A pinhole camera (सुईछिद्र कॅमेरा) is a box with a tiny hole on one side and a screen on the other. Light rays pass through the hole and form an inverted (उलटी) image on the screen. No lens is used.
In the figure : In the figure the rays from the top of the flame reach the lower part of the screen and the rays from the bottom reach the upper part, so the image is inverted.
Rule 1: Why the image is inverted — Light travels in straight lines, so rays cross at the hole.
- Easy : Top of the object → bottom of the image.
- Medium : Bottom of the object → top of the image.
- Exam-type : Left and right are also reversed.
Rule 2: Size of the hole — A smaller hole gives a clearer (but dimmer) image. A larger hole gives a brighter but blurry image.
- Easy : A pinhole of a needle gives a sharp image.
- Medium : A big hole makes a blurry image.
- Exam-type : A pinhole camera is used to view a solar eclipse safely, because we look at the image and not directly at the Sun.
Never look at the Sun directly. Use a pinhole camera for an eclipse.
Thinking a pinhole camera has a lens. It has only a tiny hole.
Memory clue: Straight rays cross at the hole → upside down.
Spot the difference
| Part | Role |
|---|---|
| Tiny hole | Lets rays pass |
| Screen | Shows the image |
| Image | Inverted |
Practice questions
- 1What type of image does a pinhole camera form?
- 2Does a pinhole camera use a lens?
- 3What kind of hole gives a clearer image?
- 4What is a pinhole camera used for?
- 5The top of the candle flame reaches which part of the screen?
- 6Why is the image upside down?
- 7What will happen to the image if the hole is made very big?
- 8Raju wants to see a solar eclipse. He looks at the Sun with a pinhole in his hand. Is that safe?
Show answers (try them yourself first)
- 1An inverted image.
- 2No
- 3A smaller hole
- 4To view a solar eclipse safely.
- 5The lower part.
- 6Light travels in straight lines, so rays cross at the hole.
- 7It becomes brighter but blurry.
- 8No. One must not look at the Sun directly; view the image on a screen.
Reflection and laws of reflection
When a ball hits a wall, it bounces back. A ray of light also bounces back when it hits a mirror or any shiny surface. That is called reflection.
Reflection (परावर्तन) is the turning back of light rays from the surface of an object. Opaque objects reflect light; transparent ones let it pass; semi-transparent ones let part pass and reflect part.
In the figure : In the figure i = r = 30°, so the angle between the incident ray and the reflected ray is 30° + 30° = 60°. The angles are measured from the normal, not from the mirror.
Rule 1: Terms — Incident ray, point of incidence, normal (perpendicular at that point), reflected ray, angle of incidence (i), angle of reflection (r).
- Easy : The ray going towards the mirror is the incident ray.
- Medium : The ray coming back is the reflected ray.
- Exam-type : The normal is a line at 90° to the mirror at the point of incidence.
Rule 2: Laws of reflection — (1) Incident and reflected rays are on opposite sides of the normal. (2) Angle of incidence = angle of reflection. (3) Incident ray, reflected ray and normal lie in the same plane.
- Easy : If i = 40°, then r = 40°.
- Medium : If i = 25°, the angle between the two rays = 25° + 25° = 50°.
- Exam-type : A ray making 30° with the mirror makes 90° − 30° = 60° with the normal, so i = r = 60° and the angle between the rays is 120°.
In the figure : In the figure a torch shines on three sheets: the transparent sheet lets all the light pass, the semi-transparent sheet lets part of it pass, and the opaque sheet lets none pass.
Reflection and laws of reflection
In the app you can move this figure with your finger.
Open in the appAngles are always measured from the normal. A ray along the normal (i = 0°) returns along the same path.
Measuring the angle from the mirror surface. Use the normal.
Memory clue: i = r, both from the normal.
Spot the difference
| Term | Meaning |
|---|---|
| Incident ray | Ray falling on the surface |
| Reflected ray | Ray turned back |
| Normal | Perpendicular at the point of incidence |
Practice questions
- 1What is reflection of light?
- 2What is the normal?
- 3State the second law of reflection.
- 4If i = 40°, find r.
- 5If i = 25°, find the angle between the incident and reflected rays.
- 6Which type of object reflects light: opaque or transparent?
- 7A ray makes 30° with a plane mirror. Find i, r and the angle between the incident and reflected rays.
- 8Anil measures the angle of incidence from the mirror surface and gets 30°. His friend says i = 60°. Who is right?
Show answers (try them yourself first)
- 1The turning back of light rays from the surface of an object.
- 2A line perpendicular to the surface at the point of incidence.
- 3Angle of incidence = angle of reflection.
- 440°
- 550°
- 6Opaque
- 7i = r = 90° − 30° = 60°; angle between rays = 120°.
- 8The friend; angles are measured from the normal: 90° − 30° = 60°.
Regular and irregular reflection
You see your face in a still pond or a steel plate, but not in a wall or a cloth. Both reflect light, but a smooth surface gives a clear picture and a rough surface does not.
Regular reflection (नियमित परावर्तन): parallel rays on a smooth surface stay parallel. Irregular reflection (अनियमित परावर्तन): parallel rays on a rough surface scatter in many directions.
In the figure : In the figure parallel rays stay parallel after reflection from the smooth surface (regular reflection) and are scattered by the rough surface (irregular reflection).
Rule 1: Regular reflection — Smooth surfaces – plane mirror, shiny metal, still water.
- Easy : You see your image in a mirror.
- Medium : A still pond gives a clear reflection of trees.
- Exam-type : A ruffled pond gives a broken reflection.
Rule 2: Irregular reflection — Rough surfaces – paper, wall, cloth, stone. The laws of reflection hold at every point.
- Easy : We see a page of a book from any side.
- Medium : We see a stone on the road because of irregular reflection.
- Exam-type : The Moon is seen because it reflects sunlight; it is not self-luminous.
Regular reflection gives images. Irregular reflection lets us see most objects.
Thinking that rough surfaces do not follow the laws of reflection. They do, but the normals point in different directions.
Memory clue: Smooth = Same direction; Rough = Random directions.
Spot the difference
| Point | Regular | Irregular |
|---|---|---|
| Surface | Smooth | Rough |
| Reflected rays | Parallel | Scattered |
| Example | Mirror | Paper, wall |
Practice questions
- 1What is regular reflection?
- 2What is irregular reflection?
- 3Give two examples of surfaces for regular reflection.
- 4Give two examples of surfaces for irregular reflection.
- 5Why can we see a book from any side?
- 6Why do we see the Moon?
- 7Do laws of reflection hold for rough surfaces? Explain.
- 8Meena says a ruffled pond gives no reflection since water is smooth. Correct her.
Show answers (try them yourself first)
- 1Parallel rays reflected by a smooth surface stay parallel.
- 2Parallel rays reflected by a rough surface scatter in many directions.
- 3Plane mirror and still water.
- 4Paper and cloth.
- 5Its rough surface scatters light in all directions.
- 6It reflects sunlight; it is not self-luminous.
- 7Yes, at every point; but the normals face different directions so the rays scatter.
- 8Waves make the surface uneven, so the reflection is broken and unclear.
Quick revision
- Light travels in a straight line and needs no medium.
- Pinhole camera: inverted image.
- Angle of incidence = angle of reflection.
- Rough surface: irregular reflection.