Binoculars

Binoculars are a type of telescope that allows a user to view far away objects using

both eyes. This requires the use of two separate telescopes, one for each eye, thus

allowing binocular vision. Structure of binoculars

Modern binoculars consist of two-barrel chambers with an objective lens,

eyepiece lens, and a pair of prisms inside whereby;

• Objective lens is used to collect light from the distant object that the user is

looking at and bringing the collected light into focus in the Eyepiece lens, which

creates a visible and magnified image.

• A pair of prisms which is used to invert the image so that it can be seen properly

upright by the eyes (ie,. without prisms in the binoculars, the observer would see

an inverted image). The prisms are made of crown glass and serve as

correcting mirrors. As the light beam passes through the prisms, multiple

reflections flip the upside-down and inverted image projected by the objective

lens, so the observer can see a normal-looking image.

Mode of action of binoculars

 When rays of light passing through the objective lens form an inverted image of

the distant object, the arrangement of the prisms rotates it by 180⁰and the

eyepiece, in turn, produces a magnified image of the initial image, enabling the

observer to easily view that object with clarity.

This happens for both the left and right tubes of the binoculars

Magnification of binoculars

• Since binoculars are made of two identical refracting telescopes, the magnifying

power of the binoculars can be calculated as in the case of the astronomical

telescope. That is;

• Whereby, 풇_풐is the focal length of the objective lens and 풇_풆is the focal length of

the eyepiece Uses of binoculars

Binoculars are used for a variety of activities.

• Binoculars are used by tourists to view animals

• Binoculars are used by soldiers to view the enemy camp from a far point

• Binoculars are used by surveyors to survey and observe the landscape and

other features

Worked examples

1. A binoculars acting as a telescope, produces an angular magnification of 7.5.

What is the eyepiece’s focal length if the binoculars have an objective lens of

focal length 75.0cm?

ANS; Given;

,

from;

cm

2. An 8.0 x binocular has 2.8 cm focal length eyepieces. What is the focal length of

the objective lenses? [ANS;

Projection Lantern

• Projection lanterns are used to display a large image on a screen.

• A projection lantern forms images of nearby slides or cinema pictures onto a

distant screen. The slide to be projected is inverted and highly illuminated.

• Apart from the illumination of the slide the optical system of the lantern projector

is very much similar to that of the camera with the direction of the light reversed.

Structure of a projection lantern

• A slide projector consists of four main parts, a projection lamp, a concave

reflector, a condensing lens and a projector lens.

• The projection lamp is placed at the centre of curvature, C of the concave

reflector so that all the light towards the reflector can be reflected back to the

same path of the condenser lens. The fan helps to cool the lamp otherwise

it would get very hot

• The condenser lens which consists of two plano-convex lenses separated by

a heat filter. The heat filter is to protect the slide from being overheated by the

lamp. The condenser is a doublet in order to reduce chromatic aberration.

• The projection lamp is also placed at the focal point of the condenser lens. The

direct light from the lamp and the reflected light from the concave reflector

spread out evenly over the surface of the slide after passing through the

condenser lens.

• The movable projector lens focuses a sharp, inverted and magnified image

onto a screen.

• The slide which has to be placed upside down in order to form an upright

image on the screen.

• Fig. below shows the optical arrangement of the projection lantern.

Mode of action of a projection lantern

• When light from the projection lamp passes through the slide and projector

lens, the resulting image is enlarged and projected onto a perpendicular flat

screen where the audience can view the reflection of the slide.

• The slide being the object, is placed between f and 2f from the projector lens,

where f is the focal length of the projector lens. The image formed on the

screen is real, inverted and magnified.

• For an image of higher magnification to be formed on the screen, the slide is

placed closer to f and the screen is placed further.

Magnification

 It is given by

Worked Examples:

1. A projection lantern is used to give the image of a slide on a screen. If the image

is 24 times as large as the slide and the screen is 72 m from the projecting lens,

what is the position of the slide from the lens Soln:

Given: m = 24, v = 72 m, u =?

From:



∴ 풕풉풆 풔풍풊풅풆 풊풔 ퟑ풎 풇풓풐풎 풕풉풆 풍풆풏풔

2. A lantern projector using a slide of 2cm x 2cm projects a picture 1m x 1m onto a

screen 12m from the projection lens. How far from the lens must the slide be?

Find the approximate focal length of the projection lens.

Soln:

Given: H_O= 2 cm, H_I= 1 m = 100 cm, v = 12 m =1200cm, u =?, f =?

From:

Also:

∴ 풕풉풆 풔풍풊풅풆 풎풖풔풕 풃풆 ퟐퟒ 풄풎 from the lens and the approximate focal length is 23.5

cm

Uses of Projection Lantern

✓ Projection of films, slides and transparencies.

✓ Projection of opaque objects, i.e. episcopic projection.

✓ In searchlights and headlights.

✓ In physical experiments such as projection of the spectrum, polarisation

experiments and interference experiments.

✓ Projection of minute objects, i.e. the projection microscope.

Lens Camera

 The lens is the image-forming device on a camera.

Structure of a lens camera

The important parts of the camera are:

(i) The converging lens that forms a real image of the object to be

photographed. This image is diminished and inverted. The lens can be moved

back and forward with the help of the focusing ring so that objects at different

distances can be brought to focus. A focused image is located on the film or

plate

(ii)The shutter controls whether light is allowed to reach the film or not. The

speed of the opening and closing of the shutter is used to control the exposure

time when taking a photograph.

(iii) The diaphragm which controls the amount of light falling on to the film per

second by using more or less of the aperture of the lens. The aperture of the

camera lens is usually described by the f-number or relative aperture. The

larger the number the smaller the aperture. Aperture is used to controls the

amount of light that will reach the camera's image sensor