2026 Science Chapter 4 (Separation Techniques) & Chapter 5 (Ray Model of Light)

Exploring diversity of matter using separation techniques o How to choose the appropriate separation technique(s) Applications: ▪ Evaporation is used to extend the shelf life of food such as ikan bilis and reduce food wastage ▪ Chromatography is used in food testing to ensure that food does not contain harmful substances and is safe for consumption ▪ In Singapore, filtration and evaporation are used in the recycling of used water to obtain potable water o The different constituents of mixtures retain their physical properties o The differences between these physical properties allows us to choose the separation technique(s) for separating the constituents of the mixtures o Other factors that should be considered when choosing a separation technique includes: ▪ Cost ▪ Reliability ▪ Environmental impact ▪ Example: Reverse osmosis helps us to obtain potable water in Singapore, however it is costly, therefore it may not be possible to use only reverse osmosis to obtain potable water o Separating a mixture of magnetic and non-magnetic substances ▪ Magnetic substances such as iron and steel are attracted to a magnet. ▪ Steel is a mixture of iron and other elements ▪ Other substances such as plastic and paper are non- magnetic ▪ Magnets can be used to separate magnetic substances from non-magnetic substances (Example: When a magnet is places near a mix of iron filings and sulphur, only the iron filings are attracted, the non-magnetic sulphur is left behind ▪ Magnetic attraction is used to separate magnetic and non-magnetic substances (Example: In Singapore, scrap metal is a waste material generated. Scrap metal contains a mixture of metals such as aluminium, copper and iron. Iron, being magnetic, can be removed from the mixture by passing a strong magnet over the scrap metal, the iron can then be reused or recycles, helping to reduce waste ▪ Separating a mixture of particles

Ray Model of Light - Used to explain why and how we are able to see things - Luminous objects emit light in all directions - The light rays travel in straight lines from the objects to our eyes, allowing us to see the object - Light from a light source can bounce off an object and enter our eyes Reflection - The bouncing of light is called reflection - Light travels in straight lines even when bouncing off objects (no matter what) - Incident ray: Ray that hits the reflecting surface - Angle of incidence (i) is the angle between the normal and incident ray - Normal: The line perpendicular to the surface at the point of incidence - Point of incidence: Point where the incident ray, normal and angle of reflection meet - Angle of reflection (r): Angle between the reflected ray and normal - Reflected ray: Light ray travelling away from the reflecting surface after bouncing off the surface - Angle of incidence (i) and the angle of reflection (r) are and will always be equal - - Mirrors: o Plane mirrors (Flat mirror): o Properties ▪ Virtual ▪ Upright ▪ laterally inverted ▪ image is same distance from the mirror as the object is from the mirror o Applications: ▪ Bathroom mirror ▪ periscope o Convex mirror (Curved outwards): o Properties ▪ Shows more things ▪ decreases the size of objects reflected in them o Applications: ▪ Funhouse mirror ▪ road corners ▪ rear-view mirrors

▪ side-view mirrors o Concave mirror (Curved inwards): o Properties ▪ Shows less things ▪ increases the size of objects o Applications: ▪ Vanity mirror ▪ ▪ dentist mirror ▪ car headlights (reflect light to form strong beams of light) ▪ microscope Refraction - A clear image can only be formed if the reflecting surface is smooth - When parallel rays of light on a rough surface, each ray is reflected in a different direction - Reflection is the bending of light at the boundary as it passes from one medium to the other => due to change of speed as it passes through 1 medium to another - Optical mediums have a property called optical density that affects the speed that light travels at through the medium - Light travels faster in air than in glass o when a ray of light travels from air (optically less dense substance) into the glass block (optically less dense substance) => it slows down and bends towards the normal o When the ray of light is exiting the glass block => gains speed and bends away from the normal - Refraction also makes things appear shallower than they actually are (like in a swimming pool) - White light is made up of a spectrum of the 7 primary colours => namely: o Red o Orange o Yellow o Green o Blue o Violet o ROYGBIV - Light bend when it travels from air to water due to its decrease in speed, after rain => moisture in the air => sunlight crosses the air-water boundary to enter each raindrop => light of each colour bends at different angles, violet rays bending the most and red rays bending the least => sunlight get separated into the seven colours of the spectrum => this is known as dispersion - Dispersion also occurs when white light passes through a glass prism

Impact of Radiation Applications - We can only see electromagnetic (EM) radiation - We cannot see some electromagnetic (EM) radiation such as: infrared and ultraviolet (UV) radiation - Infrared light: o Applications ▪ Infrared technology can be used in healthcare • In airports, public spaces to screen people for fever • Detects infrared radiation emitted by a person to form an image of their body temperature o Harmful effects: ▪ Overexposure can harm us ▪ Commonly used in welding, cutting and brazing operations as well as in furnaces and molten metal production in industries ▪ Workers exposed to prolonged exposure to infrared radiation have to wear google to protect their eyes ▪ Infrared radiation also contributes to climate change, temperatures on earth have also increased and weather patterns have changed in ways that could harm living things - Ultraviolet (UV) radiation: o Applications ▪ Exposure to a small amount => increases the production of vitamin D in the body ▪ Used in medical treatments for skin diseases like psoriasis ▪ Reproduces action of sunlight in a scientific and controlled manner ▪ UV light sterilisation is an environmentally friendlier method of disinfecting medical equipment, food and water without using harmful chemicals or releasing harmful by-products o Harmful effects: ▪ Overexposure can harm eyes and cause skin cancer ▪ Encouraged to put on sunscreen => helps filter out UV radiation - Visible light: o Applications ▪ During the day, plants need the energy provided by visible light to make sugars using carbon dioxide and water using photosynthesis ▪ Allows us to carry out our daily activities o Harmful effects: ▪ Causes chemical changes in some materials => deterioration of their quality ▪ In museums, care is taken to protect very old documents, photographs and paintings => photography

is not allowed, some objects are stored in the dark to prevent discolouration by light ▪ Too much light => light pollution => negatively impacts animals that depend on the cycle of night and day to survive (migration, reproduction, searching for food, sleeping, escaping from predators) ▪ Also uses a lot of electricity (wasteful) ▪ Can avoid this with prudent use of energy-efficient lighting technology such as LEDs and compact fluorescents (CFLs), can reduce light pollution and energy usage