## An Elementary Treatise on Geometrical Optics |

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achromatic angle of incidence angle of refraction angular astronomical telescope called centre CHAPTER circle coincide concave lens concave mirror cone consider converge convex lens critical angle diameter direction dispersive power diverge edge ellipse emergence equal equation eye-glass eye-lens eye-piece field of view Find the position focal length formula geometrical focus glass Hence illumination image formed inches incident ray index of refraction infinite distance internal reflection last Article lenses luminous point magnifying power minimum deviation normal object-glass parallel rays pass pencil of light pencil of parallel perpendicular plane mirror portion primary plane prism produced prove radius ratio ray of light real image reflected ray reflecting surface reflecting telescopes refracted pencil refraction or reflection refractive index respectively right angles screen second medium secondary focus small pencil spectrum spherical surface straight line subtended total internal reflection triangle velocity vertex whence

### Popular passages

Page 5 - ... considerable, the angle between the refracted ray and the normal to the horizontal surface of the water is not greater than the critical angle, the light can emerge and casts a bright patch on a screen placed to receive it. Tilt the mirror so as to decrease the angle of incidence on the first face. Fig. 43. The angle which the refracted ray makes with the normal to the horizontal surface is increased and can be made greater than the critical angle. The light ceases to emerge at the top of the...

Page 59 - When a ray of light passes from one medium to another, the sine of the angle of incidence is in a constant ratio to the sine of the anglo of refraction.

Page 46 - A pencil is incident obliquely on a spherical refracting surface at an angle whose tangent is equal to the refractive index of the sphere.

Page 176 - That the sum of the focal distances of any point on the ellipse is constant and equal to.

Page 32 - The locus of the image of a luminous point reflected in a plane mirror is a circle. Prove that the mirror always touches a conic section or passes through a fixed point.

Page 127 - The focal length of the object-glass of an astronomical telescope is 40 inches, and the focal lengths of four convex lenses forming an erecting eye-piece are, respectively, ^, J, f, f inches, reckoning backwards from the object-glass.

Page 108 - The angular magnification of a telescope is defined as the ratio of the angle subtended at the eye by the final image /', to the angle subtended at the (unaided) eye by the object.

Page 57 - A plane and a concave mirror are placed opposite one another on the same axis at a distance apart greater than the radius of the mirror : a person standing with his back to the plane mirror, but close to it, observes the three brightest images of a candle he holds in his hand : he moves the candle forward, till it coincides with the nearest image, prove that the other two images will coincide also at the same time. He...

Page 56 - Show that when an eye is placed to view any image formed by successive reflections at two mirrors, the apparent distance of the image from the eye is equal to the distance actually travelled by the light in coming to the eye from the original point of light.

Page 13 - The area of any triangle is equal to the radius of the circumscribing circle multiplied by half the perimeter of the triangle formed by joining the feet of the perpendiculars drawn from the angles of the given triangle to the opposite sides.