The image produced by one lens serves as the object for the next lens. Lenses in Combination The image produced by one lens serves as the object for the next lens.

eyeball: sphere with a ~ 25 mm. The Human Eye n=1.34 n=1.38 n=1.33 eyeball: sphere with a ~ 25 mm. Cornea: membrane covers the aqueous humor (liquid) iris: colored portion of the eye and controls the amount of light that enters pupil: opening in the iris, diameter 2-7 mm (dilates in dim light, contracts in bright light) the lens: flexible, shape controlled by the ciliary muscle. Inside: jelly-like substance (the vitreous humor) Retina: light-sensitive part of the eye, consisting of millions of structures called rods and cones. From the retina, electrical impulses are sent to the brain via the optic nerve Finally the brain interprets the image.

Focal length accommodates to form a sharp image at the retina The Human Eye Focal length accommodates to form a sharp image at the retina The lens only contributes about 20-25% of the refraction, but its function is important.

NEARSIGNTEDNESS objects beyond the “far point” can’t be seen sharply The Human Eye NEARSIGNTEDNESS objects beyond the “far point” can’t be seen sharply object for the second (eye) lens The lens creates an image of the distance object at the far point of the nearsighted eye.

Example 12 Eyeglasses for the Nearsighted Person The Human Eye Example 12 Eyeglasses for the Nearsighted Person A nearsighted person has a far point located only 521 cm from the eye. Assuming that eyeglasses are to be worn 2 cm in front of the eye, find the focal length needed for the diverging lens of the glasses so the person can see distant objects.

The Human Eye

FARSIGNTEDNESS objects closer than the “near point” The Human Eye FARSIGNTEDNESS objects closer than the “near point” can’t be seen sharply object for the second (eye) lens The lens creates an image of the close object at the near point of the farsighted eye.

THE REFRACTIVE POWER OF A LENS – THE DIOPTER The Human Eye THE REFRACTIVE POWER OF A LENS – THE DIOPTER Optometrists who prescribe correctional lenses and the opticians who make the lenses do not specify the focal length. Instead they use the concept of refractive power.

Angular Magnification and the Magnifying Glass The size of the image on the retina determines how large an object appears to be. Same angle apparently have the same size

Angular Magnification and the Magnifying Glass for small angles

Angular Magnification and the Magnifying Glass Example 14 A Penny and the Moon Compare the angular size of a penny held at arms length with that of the moon. Penny Moon the penny looks larger than the moon

Angular Magnification and the Magnifying Glass increases the angle the angle can be increased without a glass, moving the object closer to the eye. But the closest position is N (near point) Angular magnification with glass without glass Angular magnification of a magnifying glass

Angular Magnification and the Magnifying Glass increases the angle the angle can be increased without a glass, moving the object closer to the eye. But the closest position is N (near point) If di=-N (closest distance)

Angular Magnification and the Magnifying Glass increases the angle the angle can be increased without a glass, moving the object closer to the eye. But the closest position is N (near point) If di=infinite (eye is relaxed) For best M the image is at N f has to be very small

The Compound Microscope To increase the angular magnification beyond that possible with a magnifying glass, an additional converging lens can be included to “premagnify” the object. magnifying glass L Angular magnification of a compound microscope

Optical Instruments, Compound Microscope Fig. 34-18 O close to F1 I close to F1’ do~fo Mag. Lens fo<<s s=L-fe-fo~L-fe (34-18)

Angular magnification of an astronomical telescope The Telescope Angular magnification of an astronomical telescope

Optical Instruments, Refracting Telescope I close to F2 and F1’ Mag. Lens first image (real inverted) formed at Fo Fe and Fo coincide Final image is at infinity L=fob+fey fob>>fey