Aim: To examine characteristics of a converging lens and the images they create. Also to examine the focal length and how the focus point may be found and developing rules for locating an image via ray drawings and the intersection of rays. Chromatic aberration will also be explored and why it happens.
Theory: Both lenses and mirrors have a principle axis, yet a lens has two focal points as opposed to a mirror that has only one. When considering converging lenses, the primary focal point (PF) can be found on the opposite side of the lens in regards to the light. The secondary focal point (SF) being on the same side as the light source. Focus points on thin double convex lenses are located at either side of the lens, measured from the middle of the lens itself (see below). Chromatic aberration is a problem of converging lenses that will also be explored in this experiment. A description of each image produced will also be given (attitude, type, magnification and location).
Apparatus: The main tools (apparatus) used in the experiment consist of a ray box containing cards which allow one, two, three or four rays onto a sheet of paper at a time so that rays path from the box to the mirror will be able to be traced. Also two (2) different types of converging lenses, one being cylindrical while the other being spherical, the difference being that the spherical one is used for viewing, the cylindrical one for use with the ray box. These lenses are one of many different sorts of lenses in the "lens box". The use of a ruler would also be helpful so that accurate measurements may be taken.
Method: 1. A cylindrical lens was placed on paper and traced around, a principal axis was drawn making sure that it's at rights angles to the lens. The ray box was switched on (exposing only one ray) and lined up parallel to the principle axis, a mark where the ray leaves the box was made as well as where it hits the lens (on both sides) and where the ray bisects the principle axis. After this, the ray box was removed and using a ruler, rays were accurately drawn. The focus point (f) was measured to be of length 7.3cm (7.25). 2. The lens was replaced back on a sheet of paper with ray box emitting three rays, the middle ray lined up on the principle axis, marks were made where the ray's intersected at on the principle axis and this was measured to be the primary focus point. 3. Step two was carried out but on the other side of the lens and the secondary focus point was located and marked in. 4. The ray box was then placed on the original side of the lens and three separate rays where directed towards the lens to establish the following rules: a) A ray directed parallel to the principal axis will be directed through the primary focus. b) A ray through the secondary focus will be refracted out parallel to the principal axis. c) A ray directed through the middle of the lens will be directed out parallel to it's original source. 5. A ray was then directed at the outer edge of the lens and the rays where directed just inside the focus point. Chromatic aberration took place in this case where rays of different colors were refracted out of the lens. 6. A technique was developed for defining the focus of spherical of "viewing" lenses. Below are the instructions. Place the ray box, lens and screen on a parallel axis (principal axis). Switch the ray box on and move the lens backwards or forwards until the image goes to a single point on the screen and becomes inverse if the lens is moved any further. Measure the distance from the lens to the ray box. This is the focal length. As the lens passes the focal length the image becomes inverted (turned upside down). 7. When the lens is held at arm length to view objects around the room three characteristics can be seen from the image. The image is upright, inverse and the magnification decreases. 8. a) do>2F - inverted, real, reduced, beyond F and 2F. b) do=2F - inverted, real, same size, located at 2F. c) 2FF - no image, after 2F and F.
Conclusion and Discussion In this experiment, certain care had to be taken so as to gain as accurate results as possible. Difficulties encountered included the placement apparatuses so as too have objects parallel and at right angles (eg. The lens and ray box). Also chromatic aberration was encountered, this being when a ray hits the near edge of the lens and creates rays ranging from blue light to red light that did not pass through the primary focus point. This was overcome by directing rays towards the center of the lens and also a working environment that was not disturbed (objects and apparatuses being moved or shifted). The results from early questions seemed to be accurate and without too much error. Later questions (eg. 6 were more difficult due to work space and other light sources interfering. As for chromatic aberration, accurate layout of materials and direction of rays can overcome this problem. Chromatic aberration (from the Greek word 'khroma' meaning color) causes inaccurate measurements to be made when locating the focus point and images. Integrity was maintained when comparing results.
Questions 1. Lens (a) will have a greater focal length because the rays will converge quicker when directed through lens (b). 2. I1 - Through the primary focus (PF) I2 - Almost parallel to its main ray coming from the ray box. I3 - Directed out parallel to the principle axis. 3. Examples of chromatic aberration can be seen in a room with glass and the sun shining through, the rays of sun reflect off the glass and cause a "rainbow" to be seen on a surface (ie. the desk). 4. The image is real because it is located behind the lens and light rays are reflecting off it into our retina. 5. 1) - Place it against text and move it back until the text becomes clear and sharp, measure the distance from the text to the magnifying glass (nb: Be sure to keep the distance between you eye and the magnifying glass the same). 2) - Direct a ray through the lens and project it on a screen, move the magnifying glass until the rays coming from the light source are directed to a single point. Measure the distance from the screen to the lens.