U2.3 Lenses Flashcards
What is a convex lens
They are lenses that are thicker in the middle. they cause parallel rays of light to converge towards a point on passing through them
What is a concave lens
A lens that is thicker at the edges.
What is the principal axis
The line joining the centres of curvature of the two faces of the lens.
What is the principal focus of a convex lens
The point on the principal axis towards which all rays parallel to the principal axis converge after passing through the lens.
What is the principal focus of a concave lens
The point in the principal axis from which all rays parallel to the principal axis appear to diverge after passing through the lens
What is the focal length
The distance from the centre of the lens to the principal focus
What is the definition of a virtual image
It cannot be formed on a screen and the rays of light do not actually meet at the image.
What is the definition of a real image
It can be formed on a screen and the rays of light meet at the image
What is the lens formula
1/u + 1/v = 1/f
U= object distance
V= image distance
F= focal length
What is the equation for magnification
Magnification = hight of image / hight of object
m = hi / ho
What is the other equation for magnification
Magnification = image distance / object distance
m = v / u
What is the equation for lens power
Power of a lens (in D) = 1 / focal length (in m)
Power = 1/f
What is lens power measured in
Dioptre (D)
What are the characteristics of a normal eye
-Far point is infinity
-Near point is 25cm
What is myopia (short sight)
Near objects can be seen clearly (i.e. 25cm near point)
Distant objects are blurred due to image being formed in front of the retina
What is the cause of myopia
The eyeball is to long for the focal length of the lens or the lens is to strong for the length of the eyeball
How is myopia corrected
A concave lens is used ti produce a virtual image at the eyes far point this image then acts as an object in the eye
What is hypermetropia (long sight)
Distant objects can be seen clearly (i.e. the far point is infinity)
Close objects are blurred because the image is formed behind the retina. Hence the near point is further from the eye.
What is the cause of hypermetropia
They eye is to short for the focal length or the lens is to weak for the length of the eyeball.
How is hypermetropia corrected
A convex lens is used to produce a virtual image at the eyes near point. This image then acts as an object for the lens in the eye
