what are these?
A passive component that decreases resistance with respect to receiving luminosity on the component's sensitive surface..
Photoresistor
A photoresistor (acronymed LDR for Light Decreasing Resistance, or light-dependent resistor, or photo-conductive cell) is a passive component that decreases resistance with respect to receiving luminosity (light) on the component's sensitive surface. 
what is thallium? A chemical element. what are its characteristics? A thallium atom has 81 electrons arranged in the configuration [Xe]4f5d6s6p. It is malleable and sectile enough o be cut with a knife at room temperature. It has a metallic luster. how many isotopes does it have? 41. how is it produced? It is obtained as a by-product of smelting of copper, zinc, and lead ores. is it used in medicines? It is used for stress tests for risk stratification in patients with coronary artery disease. what are the symptoms of the disease that you just mentioned? Angina, shortness of breath, sweating , nausea or vomiting and lightheadedness. can the element you mentioned before be used in electronics? Yes, it is used in photoresistors.
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Photoresistor [SEP] Introduction
A photoresistor (acronymed LDR for Light Decreasing Resistance, or light-dependent resistor, or photo-conductive cell) is a passive component that decreases resistance with respect to receiving luminosity (light) on the component's sensitive surface. The resistance of a photoresistor decreases with increase in incident light intensity; in other words, it exhibits photoconductivity. A photoresistor can be applied in light-sensitive detector circuits and light-activated and dark-activated switching circuits acting as a resistance semiconductor. In the dark, a photoresistor can have a resistance as high as several megaohms (MΩ), while in the light, a photoresistor can have a resistance as low as a few hundred ohms.