Grade 7 Q&A: Chapter 17: Effects of Light

Concept Questions

Q1: What is meant by the rectilinear propagation of light? Give an example.

Answer: Rectilinear propagation of light means that light travels in straight lines. An example is the formation of shadows, where an opaque object blocks light, creating a dark area behind it because light cannot bend around the object.

Q2: Define reflection of light. What are the two types of reflection?

Answer: Reflection of light is the bouncing back of light when it strikes a surface. The two types of reflection are:

• Regular Reflection (Specular Reflection): Occurs from smooth, polished surfaces, forming clear images.
• Irregular Reflection (Diffuse Reflection): Occurs from rough or uneven surfaces, scattering light in various directions and not forming clear images.

Q3: State the two laws of reflection.

Answer: The two laws of reflection are:

1. The incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.
2. The angle of incidence ($\angle i$) is equal to the angle of reflection ($\angle r$). ($\angle i = \angle r$)

Q4: List four characteristics of an image formed by a plane mirror.

Answer: An image formed by a plane mirror is:

1. Virtual: Cannot be obtained on a screen.
2. Erect: Upright, not inverted.
3. Laterally Inverted: Left appears right and vice-versa.
4. Same Size: Same size as the object.
5. Same Distance: Formed as far behind the mirror as the object is in front.

Q5: What is refraction of light? Give an everyday example.

Answer: Refraction of light is the bending of light as it passes from one transparent medium to another (e.g., from air to water). This bending occurs because light changes its speed. An everyday example is a spoon appearing bent when placed in a glass of water.

Q6: How does light bend when it passes from a rarer medium to a denser medium, and vice-versa?

Answer:

• When light passes from a rarer medium (e.g., air) to a denser medium (e.g., water), it bends towards the normal.
• When light passes from a denser medium (e.g., water) to a rarer medium (e.g., air), it bends away from the normal.

Q7: Differentiate between a convex lens and a concave lens.

Answer:

• Convex Lens (Converging Lens): Thicker in the middle, thinner at the edges. It converges (brings together) parallel light rays. Used in magnifying glasses and for correcting farsightedness.
• Concave Lens (Diverging Lens): Thinner in the middle, thicker at the edges. It diverges (spreads out) parallel light rays. Used in peepholes and for correcting nearsightedness.

Q8: What is dispersion of light? Name the colors formed in a spectrum.

Answer: Dispersion of light is the phenomenon where white light splits into its constituent seven colors when it passes through a transparent medium like a prism. The band of seven colors is called a spectrum. The colors are Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR).

Q9: How is a rainbow formed?

Answer: A rainbow is formed due to the dispersion of sunlight by tiny water droplets present in the atmosphere after rainfall. These water droplets act like tiny prisms, splitting the white sunlight into its seven constituent colors, which then appear as a rainbow.

Q10: Explain why an object placed in water appears shallower than it actually is.

Answer: This phenomenon is due to the refraction of light. When light rays from the submerged object travel from water (denser medium) to air (rarer medium), they bend away from the normal. When these refracted rays reach our eyes, our brain traces them back in straight lines, making the object appear to be at a higher position (shallower) than its actual depth.

Exercise Solutions (from Page 117)

1. Fill in the blanks:

a. Light travels in a straight line.

b. The bouncing back of light from a surface is called reflection.

c. A convex lens converges light rays.

d. The splitting of white light into its constituent colors is called dispersion.

e. A rainbow is a natural example of dispersion of light.

2. Match the pairs:

a. Plane mirror - Virtual image

b. Convex lens - Magnifying glass

c. Concave lens - Peephole

d. Rectilinear propagation - Shadow formation

e. Spectrum - VIBGYOR

3. State whether the following statements are true or false:

a. Light travels faster in water than in air. False (Light travels slower in denser medium like water).

b. A concave lens is a converging lens. False (A concave lens is a diverging lens).

c. The image formed by a plane mirror is always real. False (It is always virtual).

d. The angle of incidence is equal to the angle of reflection. True

e. White light is made up of seven colors. True

4. Answer the following questions:

a. What is reflection of light? Explain with an example.

Answer: Reflection of light is the phenomenon where light rays bounce back after striking a surface. The surface from which light bounces back is called a reflector.
Example: When you look into a plane mirror, you see your image. This happens because light rays from your body strike the smooth surface of the mirror and bounce back into your eyes, forming an image.

b. Describe the characteristics of an image formed by a plane mirror.

Answer: The image formed by a plane mirror has the following characteristics:

• Virtual: It cannot be projected onto a screen.
• Erect: It is upright, just like the object.
• Laterally Inverted: The left side of the object appears as the right side in the image, and vice-versa.
• Same Size: The size of the image is the same as the size of the object.
• Same Distance: The image is formed as far behind the mirror as the object is in front of it.

c. What is refraction of light? Give two examples of its effects.

Answer: Refraction of light is the bending of light as it passes from one transparent medium to another (e.g., from air to glass or water). This bending occurs because the speed of light changes as it moves from one medium to another.
Two examples of its effects are:

1. A pencil or spoon appearing bent or broken when partially immersed in water.
2. The bottom of a swimming pool or a pond appearing shallower than its actual depth.

d. Differentiate between a convex lens and a concave lens.

Answer:

Feature	Convex Lens	Concave Lens
Shape	Thicker in the middle, thinner at the edges.	Thinner in the middle, thicker at the edges.
Effect on Light	Converges (brings together) parallel light rays.	Diverges (spreads out) parallel light rays.
Also Known As	Converging lens.	Diverging lens.
Image Formed (Virtual)	Can form virtual, erect, and magnified images (e.g., magnifying glass).	Always forms virtual, erect, and diminished images.
Common Use	Magnifying glasses, cameras, correcting farsightedness.	Peepholes in doors, correcting nearsightedness.

e. Explain the phenomenon of dispersion of light. Why does it occur?

Answer: Dispersion of light is the phenomenon where white light splits into its constituent seven colors (VIBGYOR - Violet, Indigo, Blue, Green, Yellow, Orange, Red) when it passes through a transparent medium like a prism or water droplets.
It occurs because: White light is composed of different colors, each having a different wavelength. When white light enters a medium (other than a vacuum), each color travels at a slightly different speed. This difference in speed causes each color to bend (refract) at a slightly different angle. Violet light bends the most, and red light bends the least, resulting in the separation of colors into a spectrum.

f. Give scientific reasons:

i. The bottom of a swimming pool appears shallower than it actually is.

Answer: This is due to the phenomenon of refraction of light. When light rays from the bottom of the pool travel from water (a denser medium) to air (a rarer medium), they bend away from the normal. When these refracted rays enter our eyes, our brain perceives them as coming from a higher position along a straight path, making the bottom of the pool appear shallower than its actual depth.

ii. We can see objects from different angles due to irregular reflection.

Answer: This is true because of irregular (or diffuse) reflection. When light strikes a rough or uneven surface (like most objects around us), the parallel incident rays reflect in various different directions. This scattering of light ensures that some reflected rays reach our eyes regardless of our viewing angle, allowing us to see the object from different positions. If reflection were only regular, we would only see the object from a specific angle, like an image in a mirror.

iii. A pin-hole camera forms an inverted image.

Answer: A pin-hole camera forms an inverted image due to the rectilinear propagation of light (light travels in straight lines). Light rays from the top of the object pass through the tiny pinhole and travel in a straight line to the bottom of the screen. Similarly, light rays from the bottom of the object pass through the pinhole and travel in a straight line to the top of the screen. This crossing over of light rays at the pinhole results in an inverted image being formed on the screen.

Application Questions

Q11: You are trying to catch a fish in a pond with a spear. Should you aim directly at the fish, above it, or below it? Explain why.

Answer: You should aim slightly below the apparent position of the fish. This is because of refraction. Light rays coming from the fish in the water (denser medium) bend away from the normal as they enter the air (rarer medium) before reaching your eyes. Your brain traces these bent rays back in a straight line, making the fish appear shallower and closer to the surface than its actual position. To hit the actual fish, you need to aim lower than where you see it.

Q12: Why do diamonds sparkle so brightly?

Answer: Diamonds sparkle brightly due to a combination of their high refractive index and a phenomenon called Total Internal Reflection (TIR), along with dispersion. When light enters a diamond, it undergoes multiple internal reflections because the critical angle for diamond-air interface is very small. This traps the light inside, causing it to bounce around numerous times before exiting. Additionally, diamond's high dispersion causes white light to split into its constituent colors, adding to its brilliance and fire as these colors exit at different angles.

Q13: If you stand 2 meters in front of a large plane mirror, how far away does your image appear to be from you?

Answer: If you stand 2 meters in front of a plane mirror, your image will appear to be 2 meters behind the mirror. Therefore, the total distance between you and your image will be 2 meters (to the mirror) + 2 meters (from the mirror to the image) = 4 meters.

Q14: Why do car headlights use curved mirrors, and why do dentists use small curved mirrors?

Answer:

• Car Headlights: Car headlights use concave mirrors (specifically parabolic concave mirrors). These mirrors are used as reflectors because they can collect light from a bulb placed at their focus and reflect it as a strong, parallel beam of light. This parallel beam travels a long distance, illuminating the road ahead effectively.
• Dentists: Dentists use small concave mirrors. These mirrors are used to get a magnified, erect, and virtual image of the teeth when the tooth is placed within the focal length of the mirror. This allows the dentist to see a larger view of the tooth for examination and treatment.

Q15: Imagine you are looking at a whiteboard. Why can you see the writing on it clearly from different parts of the classroom, but you cannot see your reflection clearly on it?

Answer: You can see the writing clearly from different parts of the classroom because the surface of the whiteboard is rough, causing irregular (diffuse) reflection. This means light from the writing scatters in many directions, allowing it to reach your eyes regardless of your position in the room. You cannot see your reflection clearly on it because the rough surface prevents regular reflection, which is necessary for forming a clear, sharp image like that seen in a smooth mirror.