Lenses

Diverging and Converging Lenses

• Double Convex lenses focus light rays to a point on the opposite side of the lens.

• Double Concave lenses diverge light rays and have no real focus. They have a virtual focus on the same side of the lens as the light source.

Other Convex and Concave Lenses

• A

The Focus Point and Centre of Curvature• The centre of curvature of each convex surface is on the opposite

side of the lens and is approximately the same as the focus point.

Rays for Finding Images of Objects with Lenses

• A

Images With Convex Lenses

• A

Images With Convex Lenses

• A

Images With Convex Lenses

• A

Images With Convex Lenses

• A

Image Formation With a Magnifying Glass

• A

Images in a Double Concave Lens

• A

Actual Refraction by Lenses• Light rays refract through lenses by the laws of refraction.

The results are approximated by the three locating rays used to locate images.

Dispersion of Light• The refractive indices of

different colours of light vary. This means that different colours of light slow at different rates when entering the same medium with a higher refractive index. The shorter the wavelength, the greater the slowing and therefore the greater the bending.

Dispersion of Light• Dispersion is the separation of white light into its various

colours when it refracts, due to the different indices of refraction that each wavelength has. Violet light with shorter wavelengths refracts more than red light.

Shorter Wavelengths Slow and Bend More

Lens Defects: Spherical Abberation• The shorter the focal length (greater the lens curvature),

the more that a lens fails to focus light rays to a point. The solution to this defect is to make compound lenses with different materials, to use a non-spherical curvature or to lessen the lens’s curvature (a longer focal length).

Lens Defects: Chromatic Abberation• Since different colours of light refract differently, they separate when

passed through a lens and each colour forms a separate image at a different focus resulting in a confusion of various, coloured images. Chromatic abberation can be corrected by using compound lenses of different glass composition.

The Camera• To focus a camera, the lens elements are moved out (for

close-ups) or closer in (for focusing on distant objects).

Correct Exposure• In taking a picture, just a certain amount of light should be

allowed to hit the film or sensor. Too much light causes an over-exposure (picture light and washed-out) while too little light causes an under-exposure (picture dark).

One Camera Light Control: The Aperture• Cameras have an aperture (opening) in their lenses which

can be adjusted to let more or less light to the film or sensor. The size of the aperture is given by the f-stop.

One Camera Light Control: The Aperture

• Smaller f-stop numbers are larger apertures while larger f-stop numbers are smaller apertures. From one f-stop to the next highest f-stop (ex: f 5.6 to f 8), the light is reduced by ½.

Aperture Affects Depth of Field• Larger apertures (smaller f-stop numbers) produce less depth-of-field (the

depth of focus)

Telephotos have Less Depth of Field

• Telephotos with larger apertures produce less depth-of-field (the depth of focus)

Wide Angle Lenses Create More Depth of Field

• Wide angle lenses with larger apertures (smaller f-stop numbers) produce more depth-of-field (the depth of focus)

A Second Camera Light Control: The Shutter

• Cameras have a device called the shutter which opens to let light onto the film or sensor and then closes after a set amount of time has passed.

A Second Camera Light Control: The Shutter

• Unless automatically set, shutter speeds can be set from time exposure (many seconds to minutes) to 1/30 s (slowest hand-held speed) to 1/ 60 s, 1/125 s etc. Good digital cameras allow for manual shutter settings or automatic ones.

A Second Camera Light Control: The Shutter

• Like the aperture, the shutter is calibrated to increase or decrease light by a factor of 2x. Thus a shutter of 1/125 s lets ½ the light to the film or sensor that 1/60 s does.

Creative Use of Shutter Speeds• High shutter speeds “freeze” action while low shutter

speeds blur movement. Both can be used creatively.

Correct Exposure: Correct Aperture/Shutter Combinations

• An internal light meter senses correct combinations of aperture and shutter. The combination chosen depends on the creative effect desired.

Type of Film or Sensor Sensitivity

• In older cameras, different films could be used (black and white, slow or fast colour, infrared etc) for different effects. Digital cameras produce images that can be processed by computer programs to give a much wider range of effects.

Type of Film or Sensor Sensitivity

• In older cameras, different films could be used (black and white, slow or fast colour, infrared etc) for different effects. Digital cameras produce images that can be processed by computer programs to give a much wider range of effects.

The Human Eye• The eye focuses on distant and near objects by varying the

curvature of the lens with a structure called the ciliary body. To focus on near objects (ex: when reading), muscles are used to relax the ciliary body tension on the lens which allows the lens to bulge and focus on near objects. Changing the lens to focus is called eye accommodation.

The Human Eye

• The iris opens and closes, like the camera aperture, to control the amount of light allowed onto the light-sensitive retina at the back of the eye.

Images in the Human Eye• Like a camera lens, the eye lens forms a smaller, inverted,

real image on the retina at the back of the eye.

Changes in Eye Shape• The eye may become more elongated or shortened which

leads to nearsightedness and farsightedness respectively.

Lens Correction for Nearsightedness (Myopia)• Concave lenses are used to diverge light rays through the

eye lens which causes them to focus farther back on the retina.

Lens Correction for Farsightness (Hyperopia)

• A convex lens is used to converge rays more as they pass through the eye lens which causes them to focus on the retina.

Astigmatism• Astigmatism is a focus defect due to lens deformation or

cornea deformation. Vertical and horizontal light rays are focused to different points causing blur. Correction for astigmatism is through special lenses or contacts.

Two Light Receptors in the Eye

• The human eye has cone cells that require intense (bright) light to produce color images and rod cells that are sensitive to low light levels and produce only a black-and-white image.

Two Light Receptors in the Eye

• The human eye has cone cells that require intense (bright) light to produce color images and rod cells that are sensitive to low light levels and produce only a black-and-white image.

Human Night Vision is in Black and White• Human night vision relies on rods since there is not

enough light for cones to function. Thus in low light levels, humans see in black and white.

The Blind Spot: Scotoma• Where the optic nerve enters the eye, there are no receptor

cells so the image provided by the eye actually has a visual blank spot, the blind spot (scotoma). The brain uses each eye to fill in the other’s blind spot and thus persons see full images.

Experiencing the Blind Spot• To detect your blind spots, make two differently shaped

and/or colored objects about an eye distance apart on a piece of paper. Cover the left eye and look at the left object. Move the paper closer to the eye. At some point the right object will disappear as the image falls on the blind spot. This works also by covering the right eye etc.

Optical Devices: Slide Projector• In a slide projector, the slide (object) is placed between f

and 2f which creates a larger real image farther from the lens than the slide.

Optical Devices: Simple Microscope (Magnifier)

Optical Devices: Compound Microscope• Using two convex lenses, a larger real image can be produced

of a specimen. The larger real image is made to fall in between the focus and vertex of the second lens which then produces a larger virtual image of the real image. The overall magnification is the product of the two lens magnifications.

Optical Devices: Astronomical Telescope• A convex lens produces a smaller real image of some

astronomical object. This real image falls between the focus and lens centre of a second lens which produces a larger virtual image of the real image. It will be inverted but up and down do not matter in astronomy.

Optical Devices: Terrestrial Telescope• In a terrestrial telescope an extra lens or lens system is used

to take the first real inverted image and make it a real upright image which then is magnified into a virtual upright image.

Optical Devices: Newtonian Reflecting Telescope• Newton constructed a reflecting telescope to reduce the

problem of chromatic aberration that plagued telescopes of his time. Using only a single lens greatly clarified astronomical images.