How we see objects
Switch off the lamp at night and the cupboard and chair vanish from sight; switch it on and they are back.
We see an object when light coming from it enters our eyes. That light may be given out by the object itself or reflected (परावर्तित) by it. When light rays fall on a surface and turn back, it is called reflection of light (प्रकाशाचे परावर्तन).
Rule 1: Self-luminous and reflecting objects — The Sun, a lamp and the stars give out their own light. The Moon, a chair or a book do not; light falling on them is reflected into our eyes.
- Easy : A book on a table is seen because of the lamp light falling on it.
- Medium : In a dark room a torch lights the wall, and the light reflected from the wall lets us see it.
- Exam-type : The Moon is not self-luminous; it reflects sunlight, which is why it shines.
Rule 2: Terms used in reflection — The ray that falls on the surface is the incident ray, the point where it falls is the point of incidence, and the ray that goes back is the reflected ray.
- Easy : Throw a torch beam on a mirror and the beam goes off in another direction.
- Medium : A ray through the slit of a comb falls on a mirror, reflects, and shows as a line on paper.
- Exam-type : A ray falling at right angles to the mirror returns along the same path.
We see things because light enters our eyes, not just because a source exists.
Thinking that rays leave the eyes and fall on objects. Light comes from the object into the eye.
Memory clue : "Light comes in, object seen" — the object is seen when its light enters the eye.
Spot the difference
| Point | Self-luminous | Reflecting |
|---|---|---|
| Example | Sun, lamp | Moon, book |
| Light | Gives its own | Reflects |
Practice questions
- 1When can we see an object?
- 2Name one self-luminous object.
- 3What is reflection of light?
- 4How do we see the Moon?
- 5What is the point of incidence?
- 6What is the reflected ray?
- 7Why are objects not seen in a dark room?
- 8Why is the idea that rays leave the eyes wrong?
Show answers (try them yourself first)
- 1When light from it enters our eyes.
- 2The Sun (or a lamp).
- 3Light rays falling on a surface and turning back.
- 4It reflects sunlight into our eyes.
- 5The point where the incident ray falls.
- 6The ray that goes away from the surface after reflection.
- 7Because no light from them reaches our eyes.
- 8Light comes from the object to the eye; in the dark nothing is seen even with open eyes.
Laws of reflection
A ball thrown at a wall at a slant comes back at a fixed angle. Light too behaves in a regular way.
The line drawn at right angles to the mirror at the point of incidence is the normal (लंब). The angle between the incident ray and the normal is the angle of incidence (आपात कोन); the angle between the reflected ray and the normal is the angle of reflection (परावर्तन कोन).
Rule 1: The three laws of reflection — 1) ∠i = ∠r — the angle of incidence equals the angle of reflection. 2) The incident ray, the reflected ray and the normal lie in one plane. 3) The incident and reflected rays lie on opposite sides of the normal.
- Easy : If ∠i is 30°, then ∠r is 30°.
- Medium : If ∠i is 45°, the angle between the incident and reflected rays is 45° + 45° = 90°.
- Exam-type : Using pins on paper for 30°, 45° and 60°, the angle of reflection is found equal each time.
Rule 2: A ray along the normal — A ray that falls along the normal has ∠i of 0°. So ∠r is also 0° and the ray comes straight back.
- Easy : Shine a torch straight at a mirror and the light returns to you.
- Medium : At 0° the incident and reflected rays lie on one line.
- Exam-type : For a ray along the normal both angles are 0°, so their sum is 0°.
Angles are always measured from the normal, not from the mirror surface.
Calling the angle with the mirror surface the angle of incidence. It is measured from the normal.
Memory clue : "From the normal, i = r" — measure from the normal; both angles equal.
Spot the difference
| Angle | Between which lines |
|---|---|
| Angle of incidence ∠i | Incident ray and normal |
| Angle of reflection ∠r | Reflected ray and normal |
Practice questions
- 1State the first law of reflection.
- 2If the angle of incidence is 40°, what is the angle of reflection?
- 3What is the normal?
- 4If ∠i is 50°, what is the angle between the incident and reflected rays?
- 5How does a ray falling along the normal return?
- 6Which law says the rays and the normal lie in one plane?
- 7The angle between the incident and reflected rays is 90°. What is ∠i?
- 8Why is it wrong to measure the angle of incidence from the mirror?
Show answers (try them yourself first)
- 1Angle of incidence = angle of reflection.
- 240°.
- 3The line drawn at right angles to the mirror at the point of incidence.
- 450° + 50° = 100°.
- 5It returns along the same path.
- 6The second law.
- 72 × ∠i = 90°, so ∠i = 45°.
- 8Because it must be measured from the normal.
Regular and irregular reflection
In still water you see your face clearly, but throw in a stone and the picture breaks up.
Reflection from a smooth, flat surface is regular (नियमित); reflection from a rough surface is irregular (अनियमित).
Rule 1: Regular reflection — On a smooth surface parallel rays have equal angles of incidence, so the reflected rays also stay parallel and give a clear image.
- Easy : In a mirror you see yourself clearly.
- Medium : A still pond shows a clear reflection of trees.
- Exam-type : You see your face in a polished steel plate because it gives regular reflection from a smooth surface.
Rule 2: Irregular reflection — On a rough surface parallel rays meet it at different angles, so the reflected rays spread in many directions. Even so, the laws hold at every point.
- Easy : You cannot see your face clearly on a wooden table.
- Medium : Light from a wall spreads everywhere, so the wall is seen from every part of the room.
- Exam-type : A stone makes the surface rough; rays scatter and the image blurs, but the laws are still followed.
The laws of reflection hold in both kinds of reflection.
Thinking the laws are broken in irregular reflection. The surface is rough, so the directions differ.
Memory clue : "Smooth = clear" — a smooth surface gives regular reflection and a clear image.
Spot the difference
| Point | Regular | Irregular |
|---|---|---|
| Surface | Smooth | Rough |
| Reflected rays | Parallel | Scattered |
| Image | Clear | Not formed |
Practice questions
- 1On which surface does regular reflection occur?
- 2Give one example of irregular reflection.
- 3How are the reflected rays in regular reflection?
- 4Why is a reflection clear in a still pond?
- 5Why do rays go in different directions in irregular reflection?
- 6Why is a wall seen from all parts of a room?
- 7Why does the image blur when a stone is thrown in water? Are the laws followed?
- 8Rahul says a rough surface breaks the laws. What will you tell him?
Show answers (try them yourself first)
- 1On a smooth, flat surface.
- 2Reflection from a wooden table.
- 3Parallel.
- 4The surface is smooth, so regular reflection occurs.
- 5The rough surface gives different angles of incidence.
- 6Irregular reflection spreads light in all directions.
- 7The surface turns rough giving irregular reflection; the laws are still followed.
- 8At every point ∠i = ∠r; only the angles of incidence differ.
Uses: reflection of reflected light
How does a barber show you the back of your head? He uses two mirrors.
Light can be reflected several times. The kaleidoscope (बहुरूपदर्शक) and the periscope (पेरिस्कोप) both work on reflection.
Rule 1: Image of an image — The back mirror shows an image of the back of your head and the front mirror shows the image of that image. That is how you see the haircut.
- Easy : You see the Moon in water because water reflects again the light the Moon reflected.
- Medium : Sunlight reflects at the Moon, then at the water, then reaches the eye: two reflections.
- Exam-type : For two mirrors at 90° with ∠i of 35° at the first, the angle of incidence at the second is 90° - 35° = 55°.
Uses: reflection of reflected light
In the app you can move this figure with your finger.
Open in the appRule 2: Kaleidoscope and periscope — A kaleidoscope has three mirrors forming a triangle with coloured glass bits inside; many reflections make designs. A periscope has two parallel mirrors set at 45°.
- Easy : People in a submarine see above the water with a periscope.
- Medium : A kaleidoscope rarely repeats a design; designers use it for cloth and wallpaper patterns.
- Exam-type : Looking through the bottom window of a periscope you see what is in front of the top window.
The two mirrors in a periscope are parallel and set at 45°.
Thinking a periscope works with a lens. It works by reflection from mirrors.
Memory clue : "Two mirrors, 45°, parallel" — remember the build of a periscope.
Spot the difference
| Device | Mirrors | Use |
|---|---|---|
| Periscope | Two, at 45° | Seeing above water from below |
| Kaleidoscope | Three, in a triangle | Seeing designs |
Practice questions
- 1How many mirrors are in a periscope?
- 2Where is a periscope used?
- 3How many mirrors does a kaleidoscope have?
- 4Why does a barber use two mirrors?
- 5How is the Moon's image in water formed?
- 6Why do different designs appear in a kaleidoscope?
- 7Two mirrors are at 90°. If ∠i at the first is 20°, what is ∠i at the second?
- 8What happens if one mirror is removed from a periscope?
Show answers (try them yourself first)
- 1Two.
- 2In a submarine.
- 3Three.
- 4To show the back of your head in the front mirror.
- 5Water reflects again the sunlight the Moon reflected.
- 6Because of many reflections in the three mirrors.
- 790° - 20° = 70°.
- 8The light will not be turned properly and the view above will not be seen.
Angle problems on reflection
A torch beam falls on a mirror and the angle it makes with the mirror is given. From it we can find the angle of incidence.
To solve angle problems remember three things: ∠i = ∠r, the normal makes 90° with the mirror, and the angles are measured from the normal.
Rule 1: When the angle with the mirror is given — Subtract the angle with the mirror from 90°; the result is the angle of incidence. The angle of reflection is the same.
- Easy : A ray makes 30° with the mirror. ∠i = 90° - 30° = 60°, ∠r = 60°.
- Medium : The reflected ray makes 25° with the mirror. ∠r = 90° - 25° = 65°, so ∠i = 65°.
- Exam-type : If the angle with the mirror is 20°, the angle between the two rays is 2 × 70° = 140°.
Rule 2: When the angle between the rays is given — The angle between the two rays is ∠i + ∠r. Since ∠i = ∠r, half of it is the angle of incidence.
- Easy : If the angle between the rays is 80°, then ∠i = ∠r = 40°.
- Medium : If the angle between the rays is 110°, then ∠i = 55° and the angle with the mirror is 90° - 55° = 35°.
- Exam-type : If the angle is 70°, then ∠i = 35°; so the reflected ray makes 55° with the mirror.
The angle with the mirror and the angle of incidence add up to 90°.
Taking the angle with the mirror as the angle of incidence is the most common wrong answer.
Memory clue : "90 minus mirror angle" — subtract the mirror angle from 90° to get ∠i.
Spot the difference
| Given | Find | How |
|---|---|---|
| Angle with mirror | Angle of incidence | 90° - given angle |
| Angle between rays | Angle of incidence | given angle ÷ 2 |
Practice questions
- 1If ∠i = 25°, what is ∠r?
- 2A ray makes 40° with the mirror. What is the angle of incidence?
- 3If the angle between the rays is 60°, what is ∠i?
- 4The reflected ray makes 35° with the mirror. What is ∠i?
- 5If ∠i = 48°, what angle does the incident ray make with the mirror?
- 6If the angle between the rays is 100°, what is the angle with the mirror?
- 7The incident ray makes 15° with the mirror. What is the angle between the incident and reflected rays?
- 8If ∠i is 0°, what is the angle with the mirror?
Show answers (try them yourself first)
- 125°.
- 290° - 40° = 50°.
- 330°.
- 490° - 35° = 55°.
- 590° - 48° = 42°.
- 6∠i = 50°, so 90° - 50° = 40°.
- 7∠i = 75°, so 2 × 75° = 150°.
- 890° (the ray is along the normal).