(13) Light

Question 1

(D) DEFINE REFLECTION

Answer 1

Reflection is the bouncing back of a wave that strikes the boundary between two media.

Question 2

(D) State the Laws of Reflection of Light.

Answer 2

1. The angle of incidence i is equal to the angle of reflection r.
2. The incident ray, reflected ray and normal at the point of incidence all lie on the same plane.

Question 3

(D) Name 5 characteristics of the image formed due to reflection in a PLANE MIRROR

Answer 3

1. Image is virtual
2. Laterally inverted
3. Image distance is same distance as the object distance
4. Image is UPRIGHT
5. It is of the same size as the object

Question 4

(D) Differentiate between a real image and a virtual image (GIVE BOTH DEFINITIONS)
Take into account: how it is formed and whether it can be captured on screen

Answer 4

➡️A real image can be captured on a screen and is formed by the intersection of light rays from the object at the screen

➡️A virtual image cannot be captured on a screen and is formed by the intersection of light rays which APPEAR to come from the IMAGE

Question 5

(D) DEFINE REFRACTION

Answer 5

Refraction is the bending of light as it passes from one optical medium to another. This bending is due to the change in the speed of light as it passes between the media.

Question 6

When an object slows down it
A) bends towards the normal
B) bends away from the normal

When an object speeds up it
A) bends towards the normal
B) bends away from the normal

Answer 6

A

B

Question 7

Applying the general wave equation v=f入,

Velocity ➡️ depends on medium
Frequency ➡️ depends on source
Wavelength ➡️ changes according to v and or itself

When optical density increases,_________

Answer 7

V increases!!

Question 8

What if light enters 90 degrees to the normal (from glass to air or air to glass)?

Answer 8

Light ray GO STRAIGHT no deviation. But light ray will be slower in optically denser medium in this case glass

Question 9

(D) State the principle of reversibility of light.

Answer 9

A ray of light will travel along the Same path even if it’s direction of travel is reversed (applies to both refraction and reflection)

Question 10

(D) state the laws of refraction of light.

Answer 10

➡️the incident ray, refracted ray and the normal all lie on the same plane
➡️for two given media, the ratio of the sine of the angle of incidence and the sine of the angle of refraction are constant.

n= sin i/sin r

Question 11

(D) define refractive index

Also state the 2 formulae for refractive index, n.

Answer 11

The ratio of the speed of light in a vacuum to the speed of light in that medium

Formula for refractive index:

n= speed of light jn vacuum ➗speed of light in medium
= c/v

OR

n= sin i ➗sin r
= sin larger angle ➗ sin smaller angle

Air refractive index take it as 1!!

OR

n= real depth ➗ apparent depth

OR

n= 1/sin c where c is the critical angle

(one of the mediums must be air)

Question 12

When is n= sin i ➗ sin r inapplicable?

Answer 12

When i is not measured in air

Always remember
sin larger angle ➗sin smaller angle !!

Question 13

For the straw in water question, where to put the dot of the “fake” perceived straw?

Answer 13

Anywhere higher than straw. Need not measure UNLESS question states how much higher it is

Question 14

(D) Define critical angle

Answer 14

The angle of incidence in an optically denser medium for which the angle of reflection in the optically less dense medium is 90°

Question 15

(D) define total internal reflection.

Answer 15

The complete reflection of a light ray inside optically denser medium at its boundary with an optically less dense medium

Question 16

State the conditions of total internal reflection. For it to occur

Answer 16

➡️Light ray should be inside the optically denser medium heading towards the boundary with an optically less dense medium
➡️ angle of incidence, i , in the optically denser medium should be greater than the critical angle, c, of that medium

Question 17

Structure of optical fiber

Answer 17

Glass of Low refractive index outside with glass of high refractive index inside. This allows for TIR to occur many times

Question 18

(D) state the advantages of optical fibres

Answer 18

1. Can carry much more information than cooler wires due to TIR
2. Experience less signal loss
3. Lighter than copper wires
4. Because Cheaper to manufacture than copper wires

Question 19

When will images formed in a converging lens be virtual, upright and magnified?

Answer 19

When the object is at F or in front of F

Question 20

When will images formed in a diverging lens be of the same size?

Answer 20

When object is at 2F

Question 21

When will objects formed in a converging lens be inverted real and diminished?

Answer 21

When image is distant or slightly beyond 2F

Question 22

When will the image be the same side as the object with converging lens

Answer 22

When object is at F or in front of F

Question 23

(D) define principal axis

Answer 23

Horizontal line passing through optical centre C to of the lens

Question 24

(D) define optical centre

Answer 24

midpoint between the surfaces of the lens on its principal axis

Question 25

(D) define focal point

Answer 25

Point where all the rays parallel to the principal axis converge after refraction by the lens

Question 26

(D) Define focal plane

Answer 26

Plane which passes through focal point and perpendicular to the principal axis

Question 27

(D) define focal length

Answer 27

Distance between the optical centre and focal point/ principal focus of the lens

Question 28

All light rays drawn from the same point and passed through the converging lens will go back to the same point of the image formed (ie arrow head to arrow head)

Answer 28

YES!!