Light – Reflection, Refraction
One Marks Questions
Answer: The bouncing back of light from any surface is called reflection of light.
Answer: A spherical mirror whose reflecting surface is curved inwards is called concave mirror.
Answer: A spherical mirror whose reflecting surface is curved outwards is called concave mirror.
Answer: Concave mirror
Answer: The focal length of spherical mirror is the distance between the its pole and principle focus.
Answer:
i. Infront of the mirror
ii. Behind the mirror
Answer:
Radius of curvature = 2×focal length
( R = 2f )
Answer: Diminished, virtual and erect
Answer: Concave mirror ( When object is placed between the pole and principle focus of a mirror )
Answer: A positive sign in the value of magnification indicates that the image is virtual.
Answer: Convex lens
Answer: Virtual and erect
Answer: Medium A has greater refractive index.
Answer: The size of the image formed increases as the object is brought closer towards the convex lens.
Two Marks Questions
Answer: The laws of reflection are
i. The angle of incidence is equal to the angle of reflection.
ii. The incident ray, the normal to the mirror at the point of incidence and reflected ray, all lie in the same plane.
Answer:
The mirrors whose reflecting surfaces are spherical are called spherical mirrors.
Types:
i. Concave mirror
ii. Convex mirror
Answer:
• Used in torches , search-lights , vehicle headlights.
• Used in shaving mirrors to see larger image of the face.
• Used to concentrate sunlight to produce heat in solar furnace.
Answer: They form erect image also they have a wider field of view. They enable the driver to view much larger area than would be possible with plane mirror.
Answer:
• The bottom of a tank containing water appears to be raised.
• A lemon kept in water in a glass tumbler appears to be bigger than its actual size.
Answer:
Power of lens is defined as reciprocal of its focal length. Its S I unit is dioptre (D)
Three Marks Questions
Answer: The properties of image formed by plane mirror are
• The image is virtual and erect.
• The size of the image is equal to that of the object.
• The image formed is as far behind the mirror as the object is in front of it.
• The image is laterally iverted.
Answer:
i. At infinity.
ii. Between pole and principle focus of mirror.
iii. At centre of curvature of the mirror.
Answer:
a. Concave mirrors are used in vehicle to get powerful parallel beam of light. When light source is kept at principle focus of a mirror, we get parallel beam of light after reflection by concave mirror.
b. When object is kept between pole and principle focus of a mirror , the image formed is enlarged, virtual and erect.
c. The parallel rays from the sun get converged by the concave mirror.
The laws of refraction are :
The incident ray refracted ray, and the normal to the interface of two media at the point of incidence all lie in the same plane.
The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. This is also known as Snell's law of refraction.
Where i= angle of incidence and
r = angle of refraction
Answer:
i. At focus F2 , highly diminished, real and inverted.
ii. Between F2 and 2F2 , Diminished, real and inverted.
iii. At 2F2 ,same size, real and inverted.
Four Marks Questions
Answer:
i. Optical centre : The central point of a lens is its optical centre.
ii. Centre of curvature : The centre of curvature of lens is the centre of sphere of which refracting surface of lens forms a part.
iii. Aperture: The effective diameter of the circular outline of a spherical lens is called as its aperture.
iv. Principal Focus: Principal focus of a spherical lens is the point on a principal axis where the parallel rays after refraction converge or appear to diverge from.
A concave lens always forms a virtual, erect image on the same side of the object.
Image-distance v = –10 cm
Focal length f = –15 cm
Object-distance = ?
Since,
u = - 30 cm
Thus object distance is 30 cm
Magnification m =
m =
m =
m = + 0.33
The positive sign indicates that the image is erect and virtual. The image is one-third of the size of the object.
