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Light Travels Along a Straight Line
Observing Light Beams
- You can see sunlight beams through a small opening.
- Beams of light also come from:
- Headlamps of scooters, cars, and trains.
- Torches.
- Searchlights from lighthouses or airport towers.
Light Travels Along a Straight Line
- Light travels in straight lines.
Activity Example:
- Look at a candle through a straight pipe: you can see the flame.
- Look at the candle through a bent pipe: you can’t see the flame.
- This shows light travels straight.
Reflection of Light
- Light changes direction when it hits a shiny surface.
- Examples of reflective surfaces:
- Shiny steel plate or spoon.
- Surface of water.
What is Reflection?
- When light hits a mirror, it changes direction. This is called reflection.
- Any shiny surface can act as a mirror.
Activity 11.1: Reflection with a Torch
- Use a torch covered with a chart paper with three slits.
- Shine the torch on a plane mirror fixed on a board.
- Observe how the light changes direction.
- The slits’ image in the mirror shows light reflection.
Activity 11.2: Candle and Mirror
- Place a lighted candle in front of a plane mirror.
- The flame appears behind the mirror as an image.
- Move the candle and observe the image:
- The image remains upright (erect).
- The image is the same size as the object.
- Images formed by plane mirrors cannot be captured on a screen.
Activity 11.3: Object and Image Distance
- Use a chess board or draw squares on chart paper.
- Fix a mirror vertically on the middle line.
- Place a small object (e.g., pencil sharpener) in front of the mirror.
- Observe the image’s position.
- The image is the same distance behind the mirror as the object is in front.
Key Points from notes
- In a plane mirror:
- The image forms behind the mirror.
- The image is erect and the same size as the object.
- The image distance from the mirror is the same as the object distance.
Right or Left!
Observing Your Mirror Image
- When you see yourself in a plane mirror, your image looks like you but with a key difference.
Activity 11.4: Mirror and Hand Movement
- Stand in front of a plane mirror and raise your left hand.
- Your image raises its right hand.
- Touch your right ear.
- Your image touches its left ear.
- This shows that in a mirror, right and left are swapped, but the image is not upside down.
Writing and Mirror Reflection
- Write your name on a piece of paper and hold it in front of a mirror.
- Your name appears reversed.
- This is why “AMBULANCE” is written reversed on the front of ambulances.
- Drivers can read it correctly in their rearview mirrors and give way to the ambulance.
Observations with Vehicle Side Mirrors
- In the side mirror of a scooter or car, objects appear smaller than they are.
- This helps drivers see more of the area behind them for safety.
Playing with Spherical Mirrors
Observing Images with Spoons
- Paheli and Boojho’s Observations:
- Boojho sees his erect image in a stainless steel plate (acts as a plane mirror).
- Paheli sees a smaller erect image on the back of a steel spoon.
Activity 11.5: Spoon Reflection
- Outer side of the spoon:
- Image is erect and smaller.
- Inner side of the spoon:
- Image is erect and larger.
- At a certain distance, the image becomes inverted.
Types of Spherical Mirrors
- Concave Mirror: Inner surface reflects (like the inner side of a spoon).
- Convex Mirror: Outer surface reflects (like the outer side of a spoon).
Activity 11.6: Sun and Concave Mirror
- Hold a concave mirror facing the Sun.
- Adjust a paper to get a sharp bright spot.
- The paper may start burning due to the real image of the Sun.
Real vs. Virtual Images
- Real Image: Formed on a screen (like with a concave mirror).
- Virtual Image: Cannot be formed on a screen (like with a plane mirror).
Activity 11.7: Candle Flame and Concave Mirror
- Fix a concave mirror on a stand and place a lighted candle at various distances.
- Observe the image on a screen.
- Images can be real or virtual, larger or smaller depending on the distance.
Uses of Concave Mirrors
- Used by doctors and dentists to see enlarged images.
- Used in reflectors of torches and headlights.
Observations with Convex Mirrors
- Convex mirrors form erect and smaller images.
- Used as side mirrors in vehicles to see a larger area of traffic.
Activity 11.8: Candle Flame and Convex Mirror
- Repeat the candle activity with a convex mirror.
- Real image not possible; images are always virtual and smaller.
Key Points
- Concave mirrors can form both real and virtual images.
- Convex mirrors always form virtual, smaller images.
- Side mirrors in vehicles are convex for a wider view of traffic.
Images Formed by Lenses
Using a Magnifying Glass
- Magnifying Glass:
- Used to read small print and observe tiny objects.
- It’s a type of lens.
- Other Uses of Lenses:
- Spectacles, telescopes, microscopes.
Types of Lenses
- Convex Lens:
- Thicker in the middle, thinner at the edges.
- Converges light (bends inward).
- Concave Lens:
- Thinner in the middle, thicker at the edges.
- Diverges light (bends outward).
Activity 11.9: Burning Paper with a Convex Lens
- Use a convex lens to focus sunlight on paper.
- Adjust until you get a bright spot, which may burn the paper.
- Concave lenses do not produce a bright spot.
Activity 11.10: Candle and Convex Lens
- Use a convex lens to project the image of a candle on a screen.
- Move the screen to get a sharp image.
- Observations:
- Images can be real or virtual.
- Real images can be projected on a screen.
- Virtual images cannot be projected.
Concave vs. Convex Lenses
- Convex Lens:
- Can form real, magnified, or inverted images.
- Used as magnifying glasses.
- Concave Lens:
- Forms virtual, erect, smaller images.
- Used in various optical devices.
Sunlight: White or Coloured?
Observing a Rainbow
- Rainbows appear after rain when the Sun is low.
- Consists of seven colours: red, orange, yellow, green, blue, indigo, violet.
Activity 11.11: Glass Prism Experiment
- Use a prism to split sunlight into different colours on a white surface.
- Shows that sunlight is made of seven colours, called white light.
Mixing Colours to Get White Light
Activity 11.12: Newton’s Disc
- Create a disc with seven segments, each painted with a rainbow colour.
- Spin the disc fast; it appears white, showing the mixing of colours.
Key Points
- Convex lenses converge light, forming real or virtual images.
- Concave lenses diverge light, forming only virtual images.
- Sunlight is composed of seven colours, which can be split using a prism.
- Mixing these colours can recreate white light.
Chapter Summary:
- Light travels along straight lines.
- Any polished or shining surface acts as a mirror.
- A real image can be obtained on a screen.
- A virtual image cannot be obtained on a screen.
- The image formed by a plane mirror is:
- Erect
- Virtual
- Same size as the object
- Same distance behind the mirror as the object is in front
- In a mirror image:
- Left side of the object appears on the right
- Right side of the object appears on the left
- A concave mirror can form:
- Real and inverted image
- Virtual, erect, and magnified image when the object is very close
- An image formed by a convex mirror is:
- Erect
- Virtual
- Smaller than the object
- A convex lens can form:
- Real and inverted image
- Virtual, erect, and magnified image when the object is very close
- Called a magnifying glass when used to see objects magnified
- A concave lens always forms:
- Erect
- Virtual
- Smaller image than the object
- White light is composed of seven colours.
Keywords
| Serial No. | Keywords | Serial No. | Keywords |
|---|---|---|---|
| 1 | Concave lens | 8 | Prism |
| 2 | Concave mirror | 9 | Rainbow |
| 3 | Convex lens | 10 | Real image |
| 4 | Convex mirror | 11 | Rear view mirror |
| 5 | Erect image | 12 | Side mirror |
| 6 | Magnified image | 13 | Spherical mirror |
| 7 | Magnifying glass | 14 | Virtual image |
