11. Light & Optics

Question 1

Real or Virtual Image

Answer 1

• + Real image = positive – light rays passing through it and can be projected upon screen.
• - Virtual image = negative – no light rays passing through it and cannot be projected upon screen. (light behaves as though it diverges from image point but doesn’t pass through it)

Question 2

Image for diverging

mirror & lens

Answer 2

Diverging mirror (convex) & lens (concave)

Image always: REV = Reduced, Erect (up-right), Virtual

Question 3

Snell’s Law

Answer 3

Snell’s law - mathematical relationship between path taken by light ray in crossing boundary/ surface of separation between 2 contacting substances and refractive index of each.

v = velocity, λ = wavelength

n = refractive index of medium = optical density = ability of medium to bend light à higher n = greater bend toward normal N

Question 4

Refractive Index

Answer 4

↑Higher refractive index = light slows down = changes direction = leans towards Normal N.

↓Lower refractive index = light speeds up = changes direction = leans away from Normal N.

• n = 1.0 for air; n = 1.33 for H₂O; n = 1.5 for glass (at λ = 589 nm)

Figure: Light entered ‘faster’ medium = less optically dense (air) so n₂ < n₁

Question 5

Magnification Equation

Answer 5

M positive = image erect (up-right), negative = inverted

M > 1 image enlarged, M < 1 image diminished

Question 6

Convention for

diverging & converging

lenses & mirrors

Answer 6

• i & o, positive values mean real image, negative values mean virtual
• r & f, positive values mean converging, negative values mean diverging
• M positive = image erect (up-right), negative = inverted
• M > 1 image enlarged, M < 1 image diminished