Let us consider that light rays are falling on the lens from left to right. The focus point for the converging lens lies on the right side of the lens and we know that every measurement in the lens is taken from the optical center. The converging lens focal length is measured from the optical center to the focus point and the measurement is along with the falling rays therefore, it is taken as positive.

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In the case of a diverging lens, the focal point lies on the left side of the lens. And the measurement of the focal length is taken from the optical center to the focus point and this measurement is opposite to the falling rays. Therefore, it is taken as negative.

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Sign convention: any measurement taken parallel and along the light rays is taken as positive and any measurement taken parallel and opposite the light rays is taken as negative.

A person uses a spectacle lens of f = 25 cm to help focus at short distances. If her eye alone had focal length 2.1 cm (for closest vision), what is the focal length of the eye and spectacle lens combined, in cm? You may use the formula which assumes the lenses are close together. Note that the eye needs a focal length of about 1.88 cm (power of 54 dioptre) to focus on objects at 25 cm which is the typical nearpoint for a young adult.

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Using a magnifying glass, one obtains an angular magnification for a relaxed eye given by 0.25/f where f is in metre. Using a magnifier with f = 0.08 m, A person sees an image which appears to occupy an angle of 8 degrees from top to bottom. What angle (in degrees) would the actual object occupy at the person's near point?

Given the (average) near point of a normal human eye can be taken to be 25cm, what must be the separation of the cones of the human retina?