Textbook Solutions: Keep the Curiosity Alive

Keep the Curiosity Alive (Page 166-169)

Q1. Incident Angle 40°

Question: A light ray is incident on a mirror and gets reflected. The angle made by the incident ray with the normal is 40°. What is the angle made by the reflected ray with the mirror?

Solution:

1. Angle of Incidence ($i$) = $40^\circ$.
2. According to the Law of Reflection, Angle of Reflection ($r$) = Angle of Incidence ($i$).
3. So, $r = 40^\circ$.
4. The question asks for the angle made by the reflected ray with the mirror, not the normal.
5. Angle with mirror = $90^\circ - r$
6. Angle with mirror = $90^\circ - 40^\circ = 50^\circ$.

Answer: (ii) 50°

Q2. Drawing Reflected Rays

Question: Draw reflected rays for: (i) Light falling along the normal. (ii) Light falling along the normal of a tilted mirror. (iii) Mirror tilted, light falls at 20° from normal.

Solution:

• (i) & (ii): When a light ray falls along the normal, the angle of incidence is $0^\circ$. Therefore, the angle of reflection is also $0^\circ$. The light ray retraces its path (bounces straight back).
• (iii): Since the angle of incidence is $20^\circ$, the reflected ray will also be at $20^\circ$ from the normal on the other side.

Q3. Identify the Mirror (Sketch Pen Cap)

Observations:

• (i) Image is same size and erect $\rightarrow$ Plane Mirror.
• (ii) Image is erect and diminished (smaller) $\rightarrow$ Convex Mirror.
• (iii) Image is erect and enlarged (larger) $\rightarrow$ Concave Mirror.

Match:

• (i) Plane Mirror
• (ii) Convex Mirror
• (iii) Concave Mirror

Q4. Identify the Lens/Glass (Sketch Pen Cap)

Observations:

• (i) No change in size $\rightarrow$ Flat transparent glass.
• (ii) Image is enlarged $\rightarrow$ Convex Lens.
• (iii) Image is diminished $\rightarrow$ Concave Lens.

Match:

• (i) Flat transparent glass
• (ii) Convex Lens
• (iii) Concave Lens

Q5. Incidence along Normal

Question: When light is incident along the normal, which is true?

Analysis: If the ray lies on the normal line, the angle between the ray and the normal is 0. So, $\angle i = 0^\circ$. By law of reflection, $\angle r = 0^\circ$.

Answer: (ii) Angle of incidence is 0°.

Q6. Graph Sheet Reflection

Observations in Fig 10.25:

• Left Mirror: Image is enlarged. This must be a Concave Mirror.
• Middle Mirror: Image is same size. This is a Plane Mirror.
• Right Mirror: Image is smaller (diminished). This is a Convex Mirror.

Q7. Walking towards a Large Convex Mirror

Question: A woman walks towards a large convex mirror. What does she see?

Reasoning: A convex mirror always forms an erect and diminished image. As an object moves closer to a convex mirror, the image size increases slightly but remains smaller than the object and remains erect.

Answer: (iv) her erect image keeps increasing in size (but remains diminished overall compared to reality, just gets bigger than it was when she was far away). Note: Option (i) says decreasing, which is incorrect as she gets closer. Option (iv) is the most appropriate description of the visual change.

Q8. Magnifying Glass

Question: What happens when you move a magnifying glass away? Which type of lens is it?

Solution:

• A magnifying glass is a Convex Lens.
• When close to the text, it forms an erect, enlarged image.
• When moved away, the image eventually flips and becomes inverted (because the object distance becomes greater than the focal length).

Q9. Match Columns

Correct Matching:

• (i) Concave Mirror $\rightarrow$ (a) (Definition)
• (ii) Convex Mirror $\rightarrow$ (b) (Always erect/diminished)
• (iii) Convex Lens $\rightarrow$ (c) (Can invert image if far)
• (iv) Concave Lens $\rightarrow$ (d) (Always diminished)

Q10. Assertion/Reason (Traffic Mirrors)

Assertion: Convex mirrors are preferred for observing traffic behind us. Reason: Convex mirrors provide a significantly larger view area than plane mirrors.

Analysis: Both statements are true. The reason correctly explains why they are preferred (the wider field of view allows drivers to see more cars).

Answer: (i) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.

Q11. Ray Diagrams (Fig 10.27)

• Figure (a): Rays originate from O, hit M, and reflect as if coming from I. $O$ and $I$ are equidistant. This is a Plane Mirror.
• Figure (b): Rays converge to form an image $I$. The mirror curves inward. This is a Concave Mirror.

Answer: (i) Figure (a) indicates a plane mirror and Figure (b) indicates a concave mirror.

Q12. Pencil in Water

Question: Why does a pencil in a glass of water look bent/changed?

Answer: This is due to Refraction. The curved surface of the glass tumbler containing water acts as a Cylindrical convex lens. It magnifies the pencil horizontally. Additionally, light bending as it moves from water to air makes the pencil appear displaced.