U2.3 Lenses Flashcards
(16 cards)
What is the Principal Axis?
The line joining the centres of curvature of the two faces of the lens. (This line goes through the centre of the lens and cuts both faces at 90°).
What is the Principal Focus?
Convex: The point on the principle axis toward which all rays parallel to the principal axis converge after passing through the lens.
Concave: The point on the principal axis from which all rays parallel to the principal axis appear to diverge after passing through the lens.
What is Focal Length?
The distance from the center of the lens to the principal focus.
What is a Convex (Converging) Lens?
These lenses are thicker in the middle.
Fatter the lens the shorter the distance before the rays are brought together.
They have a real foci and a positive focal length.
They have a positive lens power.
What is a Concave (Diverging) Lens?
The lens is thicker at the edges and thinner in the middle.
They have a virtual foci and a negative focal length.
They have a negative lens power.
What are the characteristics of an image formed by a convex lens by an object outside 2F?
Inverted
Real
Diminished
Eg. Camera, human eye.
What are the characteristics of an image formed by a convex lens by an object at 2F?
Inverted
Real
Same size
Eg. Lens in a telescope
What are the characteristics of an image formed by a convex lens by an object between 2F and F?
Inverted
Real
Enlarged
Eg. Projector
What are the characteristics of an image formed by a convex lens by an object at F?
No image formed
Parallel light
Image formed at infinity.
Eg. Search lights
What are the characteristics of an image formed by a convex lens by an object inside F?
Virtual
Erect
Enlarged
Eg. ?
What are the characteristics of an image formed by a concave lens?
Virtual
Erect
Diminished
Eg. ?
What is the equation for magnification?
m = hi/ho
m = v/u
m = magnification
hi = image height
ho = object height
v = image distance
u = object distance
What is the lens equation?
1/u = 1/v = 1/f
u = object distance
v = image distance
f = focal length
What is the lens power equation?
P = 1/f
P = lens power
f = focal length in m
What is Myopia (short sight)?
Near objects can be seen clearly.
Distant objects are blurry.
Image is formed in formed in front of the retina hence the far point is closer than infinity.
Caused by the eye being too long or the lens being too strong (thick).
Fixed with a concave (diverging lens).
What is hypermetropia (long sight)?
Distant objects can be seen clearly.
Close objects are blurry as the image is theoretically formed behind the retina hence the near point is further from the eye than 25cm.
Caused by the eye being too short or the lens being too weak (thin).
Fixed using a convex (converging) lens.
