Lesson 10
পোহৰ-প্ৰতিফলন আৰু প্ৰতিসৰণ
Light- Reflection and Refraction
প্ৰশ্নাৱলী QUESTIONS –1 , Page No- 168
1. অৱতল দাপোণৰ মুখ্য ফ'কাছৰ সংজ্ঞা দিয়া।
Define the principal focus of a concave mirror.
Ans:- অৱতল দাপোনৰ মুখ্য অক্ষৰ সমান্তৰালকৈ আপতিত হোৱা ৰশ্মিবোৰ প্ৰতিফলনৰ পিছত মুখ্য অক্ষৰ যি বিন্দুত লগ লাগে সেই বিন্দুটোক অৱতল দাপোণৰ মুখ্য ফ'কাছ বোলে।
The point on the principal axis at which the incident rays parallel to the principal axis of a concave mirror meet after reflection is called the principal focus of the concave mirror.
2. গোলাকাৰ দাপোন এখনৰ ভাঁজ ব্যাসাৰ্ধ 20 ছেমি। ইয়াৰ ফ'কাছ দৈৰ্ঘ্য কিমান?
The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
And:- ভাঁজ ব্যাসাৰ্ধ, Radius of Curvature (R)= 20 cm
∵ R = 2f
⇒f= R/2
= 20/2
=10 cm
3.
Name a mirror that can give an erect and enlarged image of an object.
Ans:- Concave mirror give an erect and enlarged image of an object.
4. Why do we prefer a convex mirror as a rear-view mirror in vehicles?
Ans:- Convex mirror always give an erect , through diminished image and also have a wider field of view . So, we prefer a convex mirror as a rear - view mirror in vehicle .
প্ৰশ্নাৱলী QUESTIONS –2 , Page No- 171
1. 32 cm ভাঁজ ব্যাসাৰ্ধৰ উত্তল দাপোণ এখনৰ ফ'কাছ দৈৰ্ঘ্য নিৰ্ণয় কৰা।
Find the focal length of a convex mirror whose radius of curvature is 32 cm .
Ans:- Given that,
The radius of curvature of convex mirror ( R ) = 32 cm
Therefore, Focal length ( F ) = R/2
= 32/2
= 16 cm.
2. A concave mirror produces three time magnified ( enlarged ) real image of an object placed at 10 cm in front of it . Where is the image located ?
Ans:- Object distance ( u ) = -10 cm
Magnification ( m ) = 3 cm
∵ m = -v/u
30 = -v/-10
v = 3×10
v = 30 cm
প্ৰশ্নাৱলী QUESTIONS –3 , Page No- 176
1. A ray of light travelling in air enters obliquely into water. Does the lightr ray bend towards the normal or away from the normal? Why?
Ans:- We know that, when a light ray entered from a rarer medium to denser medium, the light ray bent towards the normal .
Here air is rarer medium and water is denser medium. So, if a ray of light travelling in air enters obliquely into water , the light ray bent towards normal.
2. Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vaccum is 3×10⁸ m/s.
Ans:- Refractive index of glass, n