Light

Briefly define refraction of light

The change in the direction of the light ray at the common surface of mediums, when a light ray travels from one medium to another.

Briefly define a rare medium

The medium where light travels faster. When light travels through this medium, the light ray bends away from the normal.

Briefly define a dense medium

The medium where light travels slower. When a light ray travels through this medium, it bends towards the normal.

What is Snell’s law?

What is the relationship between relative refractive index and velocities of light in two mediums?

₁n₂ = v1/v2 ; ₁n₂ → refractive index of 2nd medium relative to the 1st medium

v1 → velocity of light in medium 1

v2 → velocity of light in medium 2

Formula for absolute refractive index

n = c/v; c → velocity of light in a vacuum

v → velocity of light in a given medium

Relationship between real depth and apparent depth

n1/n2 = real depth/ apparent depth; n1 → refractive index of the medium in which the object is placed

n2 → the refractive index of the medium from which the viewer is looking at the object

State the equation for apparent displacement

d = t [ 1 - (1/n) ] ; d → apparent displacement

t → thickness of the medium

n → refractive index of the medium

At what instance would angle of incidence = angle of refraction ?

When a light ray enters and exits through the same medium

At what instance is the angle of refraction greater than the angle of incidence ?

When a light ray moves from a dense medium to a rare medium

What is the critical angle of a medium ?

The angle of incidence in the dense medium for which the angle of refraction in the dense medium is 90 degrees

Briefly define total internal reflection

The event where the incident ray is totally refracted after the angle of incidence reaches a value greater than the critical angle.

State the conditions for total internal reflection

• Light ray should travel from a dense medium to a rare medium

• Angle of incidence > Critical angle of the medium

Relationship between critical angle and refractive index

Sin C = 1 / n ; C → critical angle

n → refractive index

Critical angle of glass

41° 48’

Critical angle of water

48° 35’

State what each angle represents

• A → Angle of prism ( angle between two refractive prism surfaces)

• i₁ → angle of incidence of the light ray

• r₁ → angle of refraction of the light ray as it enters the prism

• r₂ → angle of incidence of the light ray inside the prism

• i₂ → angle of emergence of the light ray as it exits the prism

• d₁ → angle measuring the difference between i₁ and r₁ ( how much has the light ray deviated)

• d₂ → angle measuring the difference between i₂ and r₂ ( how much the light ray has deviated)

• d → total deviation ( d₁ + d₂ )

Formulas for total deviation

d = ( i₁ + i₂ ) - (r₁ + r₂ ) or d = ( i₁ + i₂ ) - A

Equation for angle of incidence at minimum deviation

i = ( D + A / 2) ; i → angle of incidence , D → minimum deviation, A → prism angle

Equation for angle of refraction at minimum deviation

r = A / 2 ; r → angle of refraction, A → prism angle

Equation for refractive index at minimum deviation

n = Sin (A+D/2) / Sin (A/2) ; n → refractive index, A → prism angle, D → minimum deviation

State the lens formula of a thin lens.

; f → focal length of the lens

v → distance from the image to the optical centre of the lens

u → distance from the object to the optical centre of the lens

Briefly explain the New Cartesian sign convention

Distance measured from the optical center in the same direction as the incident rays are considered negative while the distance taken from the optical center and are opposite to incident rays are positive.

Briefly define linear magnification

Ratio of height of the image to height of the object

State the equation for linear magnification.

m = (height of image / height of object) = I v / u I ; v→ distance of image from lens

u→ distance of object from lens

State the equation for the focal length of a combined lens.

f → focal length of the combined lens

f₁ → focal length of the 1st lens

f₂ → focal length of the 2nd lens

How do we determine the type of lens for the combined lens?

It’s determined by the type of lens with the smaller focal length

Briefly define the power of a lens.

The quantity which represents the amount of deviation produced of light passing through the lens (ability to converge or diverge light rays) is the power of the lens.

State the equation for the power of the lens.

; P → power of the lens

f → focal length of the lens in meters

What is the sign for power of a convex lens and the power of a concave lens?

Power of convex lens → +

Power of concave lens → -

What is the far point of an eye?

It’s the most distant point that the eye can focus on.

What is the focal length of the eye lens when the eye is looking at an object positioned at the far point?

Local length of the eye lens = length of the eyeball = -25mm

What is the near point of the eye?

It’s the point nearest to the eye at which an eye can see clearly

What is the least distance of distinct vision?

It’s the distance from the near point to the eye

What is the LDDV of a healthy eye?

25 cm

What is the focal length of the eye when its focusing at an object at the near point?

-22.7 mm ( minimum focal length of the eye )

Briefly state and explain the two defects of vision that can exist.

• Short sightedness → inability to view distant objects clearly due to parallel rays, from the distant object, being focused onto a point in front of the retina.

• Long sightedness → defect where nearby objects appeared blurred because the rays of light emitted from an object placed at the near point converge to a point behind the retina

How can you correct short sightedness?

We can correct it using a concave (diverging) lens, which reduces the converging power of the eye lens so that the focal length becomes longer, causing the image to form on the retina instead of before the retina.

How can you correct long sightedness?

We can correct it using a convex (converging) lens, increasing the overall converging power of the eye so the focal length of the eye becomes smaller, causing the image to form on the retina instead of behind it.

State the formula for the angular magnification of a simple microscope at normal adjustment ( when the image is at the near point)

; m → angular magnification

D → LDDV

f → focal length of the lens

State the formula for the angular magnification of a simple microscope at non-normal adjustment ( when the image is at infinity )

m→ angular magnification

D → LDDV

f → focal length of the lens

Briefly explain the relationship between angular magnification and focal length in both normal and non-normal adjustments

Angular magnification and focal length are inversely proportional.

Briefly explain how a compound microscope is made.

A compound microscope is made using two convex lenses of small focal lengths, and is used to achieve larger magnification than a simple microscope. These two lenses are the objective lens ( which is the lens closer to the object ), which forms the real, inverted, and magnified image of the object, and the eye piece ( lens closer to the eye ), which considers the image of the objective lens as the object, and acts similarly to a simple microscope to produce a final, enlarged, virtual image.

State the formula for the angular magnification of a compound microscope at normal adjustment ( when the final image is formed at the near point )

M → angular magnification of the compound microscope

m_o → magnification of the objective lens

m_e → magnification of the eyepiece

State the formula for the angular magnification of a compound microscope at non-normal adjustment ( when the final image is at infinity )

M = M_o X (D/f_e ) ; M → angular magnification of the compound microscope

M_o → magnification of the objective lens

D → LLDV

f_e → focal length of the eyepiece

Briefly explain how an astronomical telescope is created.

It’s made to make distant objects appear much larger and clearer, so it should have a large magnifying power. The astronomical telescope has two convex lenses ; objective lens → larger focal length, eye piece → smaller focal length.

State the formula for the angular magnification of an astronomical telescope at normal adjustment ( when the final image is at infinity )

M = f_o / f_e ; f_o → focal length of the objective lens, f_e → focal length of the eye piece

State the formula for the angular magnification of an astronomical telescope at non-normal adjustment ( when the final image is at the near point )

M = (f_o / D) + (f_o / f_e); f_o → focal length of the objective lens, D → LDDV, f_e → focal length of the eye piece

Briefly explain what the eye ring is in a compound microscope or astronomical telescope

The eye-ring is the best position, of the eye, to see images clearly.