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๐ฌ Think about it
A flat window shows things at their real size. But a drop of water on glass makes the letters under it look bigger — like a tiny magnifying glass. Curved transparent things bend light too. Let's meet lenses.
How does a water drop act like a lens?
- A water drop on glass has a curved outer surface.
- Letters seen through it look enlarged.
- The curved surface changes how the text looks.
- So the drop acts like a simple lens.
Example:
A magnifying glass makes small print look bigger, just like the water drop.
In this Activity, we will make a tiny lens from a water drop and look at text through it.
Materials needed
A flat strip of glass or clear plastic, a few drops of oil (or wax), a dropper, water, printed text.
Procedure
1. Spread a thin coat of oil on the glass strip.
2. Place a small drop of water on the oiled spot so it forms a round drop.
3. Examine whether the drop's surface is flat or curved.
4. Place printed text directly under the water drop.
5. Look down through the drop and note any change in the letters.
2. Place a small drop of water on the oiled spot so it forms a round drop.
3. Examine whether the drop's surface is flat or curved.
4. Place printed text directly under the water drop.
5. Look down through the drop and note any change in the letters.
Observation
The water drop has a surface curved outwards, and the letters under it look enlarged.
Explanation
The curved drop bends light and makes the text look bigger, acting like a simple lens — just like a magnifying glass.
โ Summary
- Oil coat spread
- Water drop placed
- Text underneath
- Letters enlarged
What are convex and concave lenses?
- A lens is a clear curved piece of glass or plastic.
- A convex lens is thicker in the middle than at the edges.
- A concave lens is thicker at the edges than in the middle.
- Light passes through a lens, so we see through it.
Convex lens
Concave lens
Thicker in the middle.
Thicker at the edges.
Bulges outwards.
Caves inwards.
What images do convex and concave lenses form?
- Close to a convex lens, an object looks erect and enlarged.
- Farther away, it appears inverted and then smaller.
- A concave lens always shows an erect, diminished image.
- The image size changes with distance.
Example:
A magnifying glass (convex) enlarges text held close, but flips a distant object.
In this Activity, we will look at an object through convex and concave lenses at different distances.
Materials needed
A convex lens, a concave lens, a lens holder, a small object.
Procedure
1. Place the convex lens upright in its holder.
2. Place the object behind the lens and look through from the other side.
3. Slowly move the object farther and watch how the image changes.
4. Repeat the steps using the concave lens.
5. Compare the images seen through both lenses.
2. Place the object behind the lens and look through from the other side.
3. Slowly move the object farther and watch how the image changes.
4. Repeat the steps using the concave lens.
5. Compare the images seen through both lenses.
Observation
Through the convex lens the object is erect and enlarged when close, then inverted and smaller when far. Through the concave lens it is always erect and diminished.
Explanation
A convex lens changes the image with distance and can invert it; a concave lens always gives a small, erect image.
โ Summary
- Object behind lens
- Viewed through
- Moved farther
- Images compared
How do lenses affect a beam of parallel light?
- Light passes straight through a thin flat glass plate.
- A convex lens converges the light passing through it.
- A concave lens diverges the light.
- So convex is a converging lens; concave is a diverging lens.
Parallel Light Through Lenses
Thin glass plate
Light passes straight through.
↓
Convex lens
Light converges (converging lens).
↓
Concave lens: light diverges (diverging lens)
In this Activity, we will pass parallel light through a glass plate and two lenses to see how each bends it.
Materials needed
A thin glass plate, a convex lens, a concave lens, a torch, a comb, a paper clip, two identical books, white paper.
Procedure
1. Fix the glass plate or a lens upright between two books.
2. Spread paper sheets on both books.
3. Shine multiple parallel beams onto the thin glass plate and observe.
4. Repeat with the convex lens, then the concave lens.
5. Record and analyse the path of the beams.
2. Spread paper sheets on both books.
3. Shine multiple parallel beams onto the thin glass plate and observe.
4. Repeat with the convex lens, then the concave lens.
5. Record and analyse the path of the beams.
Observation
Light passes straight through the glass plate. The convex lens converges the light; the concave lens diverges it.
Explanation
A convex lens is a converging lens and a concave lens is a diverging lens, because of how their curved surfaces bend light.
โ Summary
- Beams through glass
- Convex converges
- Concave diverges
- Lens types named
๐ก A step further
- If we draw what we observed in Activity 10.10, we get the figures shown.
- Through a thin glass plate, parallel rays pass straight through unchanged.
- Through a convex lens, parallel rays converge to a point.
- Through a concave lens, parallel rays diverge outward.
In this Activity, we will use a convex lens to focus sunlight and burn a paper.
Safety first
Do not look at the Sun directly or through the lens, as it may damage your eyes.
Materials needed
A convex lens, a sheet of paper, sunlight.
Procedure
1. Repeat Activity 10.7 using a convex lens instead of a concave mirror.
2. Hold the lens in the path of the sunrays.
3. Focus the light to a bright spot on the paper.
4. Hold steady and check whether the paper burns.
2. Hold the lens in the path of the sunrays.
3. Focus the light to a bright spot on the paper.
4. Hold steady and check whether the paper burns.
Observation
The convex lens focuses sunlight into a bright spot, and the paper begins to burn.
Explanation
Like a concave mirror, a convex lens converges sunlight to one point, producing enough heat to burn the paper.
โ Summary
- Convex lens in Sun
- Light focused
- Bright spot
- Paper burns
Where are lenses used in everyday life?
- Eyeglasses use lenses to help people see clearly.
- Cameras, telescopes, and microscopes all use lenses.
- Even our eye has a convex lens inside it.
- The eye lens can change shape to focus near or far.
Example:
A smartphone camera uses lenses to focus light and capture sharp photos.
Important Points
- A convex lens is thicker in the middle; a concave lens at the edges.
- Convex lens converges light; concave lens diverges it.
- Lenses are used in eyeglasses, cameras, and even the eye.
โ Test Yourself
-
How is a convex lens shaped?
View Answer
It is thicker in the middle than at the edges and bulges outwards. -
What image does a concave lens always form?
View Answer
An erect and diminished (smaller) image, whatever the distance. -
Which lens is a converging lens?
View Answer
A convex lens converges light, so it is called a converging lens. -
Can a convex lens burn paper?
View Answer
Yes. It focuses sunlight to a hot bright spot, which can burn the paper. -
What kind of lens is in our eye?
View Answer
A convex lens, which can change its shape to focus on near or far objects.
Important Definitions
- Lens — a transparent piece of glass or plastic with curved surfaces.
- Convex lens — a lens that is thicker in the middle than at the edges; a converging lens.
- Concave lens — a lens that is thicker at the edges than in the middle; a diverging lens.
๐ NCERT Question 4 — In Fig. 10.24, the
Match each image of a sketch-pen cap with the correct lens or glass type.
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๐ NCERT Question 8 — Hold a magnifying glass
Hold a magnifying glass over text, then move it away. What do you notice, and which lens is it?
View Answer →
๐ NCERT Question 12 — Place a pencil behind
A pencil behind a tumbler half-filled with water looks changed. Explain why.
View Answer →
