Lens Applications

Reflecting Telescope (Newtonian) The incoming rays are reflected from the mirror and converge toward point A At A, an image would be formed Instead, a small flat mirror, M, reflects the light toward an opening in the side which passes the light into an eyepiece The image formed after reflecting off of the flat mirror is then magnified by the eyepiece

Refracting Telescopes Two lenses are arranged at separation equal to the sum of their focal points Light from distant objects enters the objective lens and forms a real inverted image between the two lenses • The first image is near the focal point of the eyepiece lens

Refracting Telescopes Light from the first image then passes through the eyepiece lens and forms a virtual image that appears to come from beyond the objective lens The final image is inverted and larger than the image produced by the objective lens The distance between the two lenses, ƒo + ƒe, corresponds to the length of the tube of the telescope

Cameras A camera gathers light from large, distant objects and forms smaller, real images on film or a sensor as the light passes through a converging lens When the object is beyond twice the focal length (2F) the camera lens produces a real, inverted, and smaller image of the object on a film or digital sensor where it is recorded To get a sharp image you move the lens in and out to focus.

Magnifying Glass The simplest optical device Converging lens in which the object is located between F’ and the lens No real image is produced by the object at this location Refracted rays diverge to our eyes and we project these rays backwards, forming and enlarged, virtual image located on the same side of the lens as the object

The Human Eye

Parts of the Eye Cornea Fluid Ciliary Muscles Lens Iris Pupil Retina

The Anatomy of the Eye -Light entering the eye first meets the cornea, a thin protective membrane (refractive index n= 1.38) that refracts light before entering the eye -Light passes through a fluid to the crystalline lens (n=1.44), the shape of which can be adjusted by the ciliary muscles

The Anatomy of the Eye -The ciliary muscles adjust the length and thickness of the lens allowing the lens to focus light from both nearby and distant objects onto the retina -The back of the eye is the retina, the “screen” on which images are formed. Its detects the intensity and frequency of light and sends the information to the brain via the optic nerve.

The Anatomy of the Eye -The amount of light entering the eye can be controlled by the iris (the coloured part of the eye) which can dilate to admit more light -The black part of the eye, or the pupil, is not a proper anatomical part of the eye. It is just the opening for the light and appears black because all incident light is absorbed and not reflected

Rods and Cones -Rods & Cones are the light sensitive cells, or, photoreceptors that make up the retina -Rods are more sensitive to light and dark changes, shape, and movement but contain only one type of light-sensitive pigment and are therefore not good for color vision. Rods are more numerous than cones in the periphery of the retina.

Rods and Cones Cones are most sensitive to one of three different colours (green, red or blue). Signals from the cones are sent to the brain which then translates these messages into the perception of colour. Cones, however, are not as sensitive to light as rods. That's why you cannot see colour very well in dark places.

Colour Blindness Someone who is colour blind does not have a particular type of cone in the retina or one type of cone may be weak. In the general population, about 8% of all males are color blind and about 0.5% of all females are color blind.

Focusing problem – Hyperopia Hyperopia (Far-sightedness) Can’t see objects that are up close Eye doesn’t refract light well enough to form an image Usually occurs because the distance between the lens and the retina is too small or because the cornea-lens combination is too weak Light ends up focused behind the retina

Hyperopia

Correction for Hyperopia – Converging lens The eye needs help in refracting light A converging lens called a positive meniscus lens can be used to correct this problem

Focusing problem - Myopia Myopia (Near-sightedness) Can’t see objects that are far away Usually occurs because the distance between the lens and the retina is too large or because the cornea-lens combination converges light too strongly Light ends up focused in front of the retina

Myopia

Solution for Myopia – Diverging lens The eye needs help in refracting light A diverging lens, called a negative meniscus, helps to diverge the incoming rays so they end up focused on the retina

Focusing problem - Presbyopia lens loses its elasticity over time typically affects people who are over 40 results in a form of far-sightedness

Laser Eye surgery The cornea is re-shaped to ensure proper refraction of light Cornea needs to be flattened to decrease refractive power Increase the slopes of the cornea (make it fatter) to increase refractive power