Light | Term 3 Unit 1 | 7th Science - Numerical problems | 7th Science : Term 3 Unit 1 : Light
Chapter: 7th Science : Term 3 Unit 1 : Light
Numerical problems
Laws of reflection:
Example 1
In the figure, the incident ray makes 27° with the normal, then find the angle of reflection.
Solution:
Angle of incidence = 27°
According to the laws of refelection, the angle of refelection = Angle of incidence = 27°
Example 2:
A light ray strikes a reflective plane surface at an angle of 43° with the plane surface.
i. Find the angle of incidence.
ii. Find the angle of reflection.
iii. Find the angle between the incident and the reflected ray
iv. Find the angle between the reflected ray and the plane surface.
Solution:
We use the diagram shown below to answer the questions.
a. Angle of incidence: i = 90 -43 = 47°
b. angle of reflection r = i = 47°
c. i + r = 47 + 47 = 94°
d. x = 90 - r = 90 - 47 = 43°
Q. A person is looking at the image of a tree in a mirror placed 3.5 m in front of him. Given that the tree is at 0.5 m behind his eyes. Find the distance between the image of the tree and his eyes. What are needed to see an object?
Answer:
Distance between the person and the mirror = 3.5 m
Distance between the person and tree (object) = 0.5 m.
The image formed in the mirror = 4 m
The distance between the image of the tree and his eyes = 4 + 3.5 = 7.5 m
Things needed to see an object :
❖ Source of light
❖ Object
❖ Eyes
Q. A boy of height 1m 45 cm is standing in front of a long mirror at a distance of 2 m. From this information, fill up the following sentences:
a. The distance between the boy and his image is
b The height of the image is
c. When the boy moves 1m forward, the distance between her and her image is
Q. Draw a diagram of a pin hole camera showing the rays of light passing between the Object and its image
Answer:
Q. Two plane mirrors M1 and M2 are placed perpendicular with each other, as shown in figure. The ray AB makes an angle 39 ° with the plane mirror M1, then
1. The reflected rays are , ____________,____________
2. The incident rays are , ____________,____________
3. What is the angle of incident corresponding to the ray BC?
4. What is the angle of reflection corresponding to the ray CD
Answer:
1. BC, CD
2. AB , BC
3. ∟i =39°
4. ∟r =39°
Q. Rajan was playing with the mirror images of a clock. He looked at the clock in his room. It was showing 1:40. Draw the position of the hands on the real clock and on its mirror reflection. Write below the picture what time each picture is showing.
Answer:
Q. If a ray of light is falling on a plane mirror at an angle of 500 is formed, what will be the angle of reflection?
Answer:
Angle of incidence
∟i = 90° - 50°
∟i = 40°
According to laws of reflection,
∟i = ∟r
Angle of incidence = Angle of reflection
∟i = 40°
∴ ∟r = 40°
Angle of reflection ∟r = 40°
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