Lenses

Front 1

What is a lens?

Back 1

A lens consistsof a piece of glass or plastic, ground so that its two refractive surfaces is asegment of either a sphere or a plane. whichtransmits and refracts light, converging or diverging the beam.

Front 2

Describe Converginglenses (convex)

Back 2

Thickestinthe middle and form positive focal lengths. Light rays converge.

Front 3

Describe Diverginglenses (concave)

Back 3

Thickest atthe edges and have negative focal lengths. Light rays diverge.

Front 4

What is the optic centre?

Back 4

Optic centreis found at the centre of the lens.

Front 5

What is the principal axis?

Back 5

The principalaxis is a straight line through the optic centre which is at right angles tothe face of the lens.

Front 6

Define focal length?

Back 6

The focallength(f) isthe distancefrom the lens to the focal point. It is the image distancethat corresponds to an infinite object distance.

Front 7

How is light Focused in Convex Lenses

Back 7

Raysparallel to the axis pass through the focal point (F)after being converged by the lens.

Front 8

How is light Focused in Concave Lenses

Back 8

Raysparallel to the axis are diverged by the lens. The focalpoint(F) isthe point at where the diverging rays appear to originate.

Front 9

What is a real image?

Back 9

Whetherself luminous or diffusely reflecting every point in an object gives out lightin all direction.

A realimage is due to the actual intersection of rays and can be located on a screen.

Front 10

What is a virtual image?

Back 10

Whether self luminous or diffusely reflecting every point in an object gives out light in all direction.

Avirtual image is due to the apparent intersection of waves and cannot befocused on a screen for example

Front 11

Howan image is formed in a convex lens

Back 11

Ray 1– travels parallel to the principle axis and passes through fafterbeing refracted by the lens

Ray 2– travels through the optic center and continues in a straight line path

Ray 3– passes through the f beforebeing refracted by the lens and then travels parallel to the principle axis

Front 12

How is the Imageof a distant object in a Convex Lens formed

Back 12

If the object is a large distance awayfrom the lens (we say infinity) a real image is formed and at the focus (focalplane)

Front 13

What happens to the Imagewhen object is placed outside 2f?

Back 13

• image formedbetween f and 2f

• image isinverted and smallerthan object

Front 14

What happens to theImagewhen object is at 2f

Back 14

•real imaged formed at 2f

• image is invertedand same sizeas object

Front 15

What happens to theImagewhen object is between 2f and f

Back 15

•real imaged formed outside 2f

• image isinverted and largerthan object

Front 16

What happens to theImagewhen object is placed at f

Back 16

•parallel refracted rays never meet

• no image is formed

Imageis formed at infinity

Front 17

What happens to theImagewhen object is between f and the lens

Back 17

•the image is formed on the same side asthe object

• the image is virtual,upright and larger than object

Front 18

Howan Image is Formed in a Concave Lens

Back 18

Ray 1 – travelsparallel to the principle axis and appears to come from fafterbeing refracted by the lens

Ray 2– travels through the optic center and continues in a straight line path

Ray 3– isrefracted bythe lens and then travels parallel to the principle axis

Front 19

Howan Image is Formed in a Concave Lens

Back 19

The imageis always virtual, upright (erect) and located on the same side of the lens asthe object

The imageis always diminished.

The nearerthe object is to the lens the bigger the image is.

Front 20

Formulafor lenses

Back 20

1/u + 1/v = 1/f

u = object distance, object isalways real

v = image distance. + for real and– for virtual

f =focal length. + for convex – for concave

Front 21

Formula for magnification

Back 21

Magnification =height of image (hi) / height of object (ho)

Magnification = - image distance(v)/ object distance (u)

M = -v/u = hi/ho

When M is positive the image isupright When M is negative the image isinverted

When M is less than one the imageis smaller

When M is more than one the imageis magnified

Front 22

Powerof a lens

Back 22

The power of a lens is related to the focallength.

It is the degree to which a lensconverges or divergeslight

The power of a lens = 1/ focal length

The shorter the focal length thegreater the converging or diverging power of the lens

Front 23

What is a Magnifyingglass

Back 23

A magnifyingglass is a convex lens which produces a magnified (larger) image of an object.

Front 24

How is an image in a Magnifying glass produced?

Back 24

A magnifyingglass produces an upright, magnified virtual image. The virtual image producedis on the same side of the lens as the object.

For amagnified image to be observed the distance between the object and the lensmust be shorter than the focal length of the lens.

Front 25

What is Accommodation?

Back 25

Accommodationis the focusing ability of the eye by changing the shape of the convex lens.

Front 26

What happens When the ciliary muscle is relaxed?

Back 26

Whenthe ciliary muscle is relaxed, the lens is at its thinnest; as when focusing ondistant objects. The focal length of the lens is at its largest.

Front 27

What happens When the ciliary muscle contracts?

Back 27

Whenfocusing on near objects the ciliary muscle contracts and the lens bulgesdecreasing the focal length.

Front 28

What is the near point?

Back 28

Thenear point is the closest distance an object can be placed to the eyes for animage to be formed on the retina. The average distance is 25 cm.

Front 29

What is the far point?

Back 29

Thefar point is the farthest distance the relaxed eye can focus light on theretina and is at infinity.

Front 30

Explain a person with normal vision?

Back 30

Aperson with normal vision can focus clearly on objects both near and far.

Front 31

Explain sight problems

Back 31

Whenthe eye suffers a mismatch between the focusing power of the lens-cornea systemand the length of the eye, rays are no longer focused on the retina and blurredimages are formed

Front 32

Explain Longsighted-ness (farsighted or hyperopia)

Back 32

A personwho is long sighted can focus clearly on distant objects but cannot focus onnearby objects. The near point of the eye is much greaterthan the average 25 cm.

Front 33

How is Long sighted-ness (farsighted or hyperopia) treated?

Back 33

The focal length of a long sightedeye is greater than normal and rays from near objects are focused behind theretina. The condition can be corrected by using a convex lens which convergesthe rays before entering the eye allowing them to focus on the retina.

Front 34

Explain Shortsighted-ness (nearsighted ormyopia)

Back 34

A personwho is shortsightedcan focus clearly on near objects butcannot focus on distant objects.Thefar point of the eye is not at infinity and may be less than 1 meter.

Front 35

How is Short sighted-ness (nearsighted or myopia) treated?

Back 35

The focal length of a short sightedeye is less than normal and rays from far objects are focused in front of theretina. The condition can be corrected by using a concave lens which divergesthe rays before entering the eye allowing them to focus on the retina.

Front 36

An ophthalmologistdeterminesthat a patients eyes are shorter than normal. The ophthalmologist shoulddiagnose this patient with ______ and recommend he use ______.

Back 36

hyperopia; converging lenses